https://fabpedigree.com/james/gmat200.htm

It seems like he had two half sisters and a half brother, presumably all by his mother's second marriage

https://www.geni.com/people/Sir-Isaac-Newton/6000000001299393239

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http://www.stegen.k12.mo.us/tchrpges/sghs/aengelmann/archimedes2.htm

https://www.geni.com/people/Carl-Friedrich-Gauss/6000000000114284058

https://en.wikipedia.org/wiki/Leonhard_Euler

Bernhard Riemann

"His father, Friedrich Bernhard Riemann, was a poor Lutheran minister in Breselenz. He and his wife, Charlotte nee Ebell, had six children, out of which Georg was born second. From his childhood, he was very shy and introverted."

https://www.thefamouspeople.com/profiles/bernhard-riemann-biography-440.php, https://www.geni.com/people/Bernhard-Riemann/6000000000152092209

https://www.thefamouspeople.com/profiles/henri-poincare-448.php

Joseph-Louis Lagrange

The link says he was "the eldest of the two surviving children"

https://www.thefamouspeople.com/profiles/joseph-louis-lagrange-446.php

Euclid of Alexandria

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https://en.wikipedia.org/wiki/Euclid

https://en.wikipedia.org/wiki/David_Hilbert#Early_life_and_education

Gottfried W. Leibniz

"Gottfried was the elder of his parents’ two children, having a younger sister named Anna Katharina Leibniz Löffler. From his father’s two previous marriages, he also had five half-siblings; Anna Rosina Leibniz, Johann Friedrich Leibniz, Elisabeth Leibniz, Anna Magdalena Leibniz, and Susanna Leibniz."

https://www.thefamouspeople.com/profiles/gottfried-w-leibniz-536.php

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"Pierre had three siblings, one brother and two sisters."

"He was once thought to have been born in 1601, but this was a different Pierre de Fermat – a half-brother who died in infancy."

"Pierre had one brother and two sisters and was almost certainly brought up in the town of his birth. "

http://totallyhistory.com/pierre-de-fermat/, https://www.famousscientists.org/pierre-de-fermat/,
https://en.wikipedia.org/wiki/Pierre_de_Fermat

"Galois had a sister named Nathalie-Théodore and a brother called Alfred."

http://www-history.mcs.st-and.ac.uk/Extras/Galois_family.html, https://en.wikipedia.org/wiki/%C3%89variste_Galois, https://www.thefamouspeople.com/profiles/variste-galois-423.php

John von Neumann

https://www.famousscientists.org/john-von-neumann/

https://www.geni.com/people/Ren%C3%A9-Descartes-seigneur-du-Perron/6000000010647044004

Karl W. T. Weierstrass

"Weierstrass was the first child of Wilhelm Weierstrass, secretary to the mayor of Ostenfelde, and Theodora Vonderforst, who were married five months before his birth." This is suggestive that he was the first child of the family.

"brother Peter told Mittag-Leffler: “Karl looked awfully bad when he returned back home! It was
so painful to see my elder brother in such a state! Four years and no exam!” [Biermann, 2008]." So he had at least one younger brother.

"There, he reached a high level of mathematical competence as he regularly read Crelle’s Journal and also tutored one of his younger brothers" suggests that he had at least two brothers.

"The family soon moved to Westernkotten in Westphalia, where Weierstrass' three siblings, Peter, Klara, and Elise, were born."

https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/weierstrass-karl-theodor-wilhelm,
https://arxiv.org/pdf/1508.02928.pdf,
http://kobotis.net/math/MathematicalWorlds/Fall2016/131/Biography/Weierstrass.pdf,
http://www.bookrags.com/biography/karl-theodor-wilhelm-weierstrass-wom/#gsc.tab=0

Srinivasa Ramanujan

"His mother gave birth to several children after Ramanujan, but none of them survived infancy." Suggests that he was an only child?

Google search, gives him a brother named Sadagopan.

Geni doesn't list any siblings.

"Ramanujan was one of several siblings in a Tamil Brahmin family in Madras. But he lost all of them to a smallpox epidemic in 1889"

https://www.thefamouspeople.com/profiles/srinivasa-ramanujan-503.php,
https://www.geni.com/people/Srinivasa-Ramanujan/6000000000495602927,
https://www.indiatoday.in/education-today/gk-current-affairs/story/srinivasa-ramanujan-life-story-973662-2017-04-26

Hermann K. H. Weyl

Geni doesn't list any siblings, but other sources said that "not much was know about his childhood."

https://www.geni.com/list?focus_id=6000000000414033568&group=immediate_family,

Peter G. L. Dirichlet

"Although his family was not wealthy and he was the youngest of seven children"

"In 1815, his father decided to send Niels’ elder brother Hans to the Cathedral School in Christiania (Oslo)"

Google gives 4 sibs: 2 male, 2 female.

"Niels Henrik Abel was born in Nedstrand, Norway, as the second child of the pastor Søren Georg Abel and Anne Marie Simonsen."

https://www.thefamouspeople.com/profiles/niels-henrik-abel-6047.php,
https://www.google.com/search?ei=UzdVW_jmJcjcswXH87vgBQ&q=niels+henrik+abel+siblings&oq=niels+abel+sibl&gs_l=psy-ab.3.0.0i22i30k1.24366.25755.0.35331.5.5.0.0.0.0.144.515.0j4.4.0....0...1.1.64.psy-ab..1.4.513...0.0.L52fkKOO8Fo,
https://en.wikipedia.org/wiki/Niels_Henrik_Abel

"Georg, the oldest of six children, was regarded as an outstanding violinist"

https://www.wikiwand.com/en/Georg_Cantor

"He was the second of four children of banker Simon Jacobi. His elder brother Moritz von Jacobi would also become known later as an engineer and physicist."

https://en.wikipedia.org/wiki/Carl_Gustav_Jacob_Jacobi

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He had two sisters, Sophia and Henrietta-Caroline and two brothers, William Henry and Charles Bagot. While Sophia and William Henry were elder to him, Charles Bagot and Henrietta-Caroline were younger. He never saw William, who died in infancy. Charles Bagot grew up to a noted linguist

https://www.thefamouspeople.com/profiles/arthur-cayley-512.php

"She was the eldest of four children, but one of only two that survived childhood. Her brother, Fritz also made a career of mathematics."

https://www.agnesscott.edu/lriddle/women/noether.htm

Pythagoras of Samos

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Pythagoras was born in the eastern Aegean island of Samos, Greece in 570 BC. It is believed that his mother, Pythias, was a native of the island while his father, Mnesarchus, was a merchant from Tyre (Lebanon), dealing in gems. It is also said that he had two or three siblings.

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Leonardo `Fibonacci'

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From a legal document, we know that his father was called Guilichmus, which translates as “William” (the variant Guilielmo is also common) and that he had a brother named Bonaccinghus.

William R. Hamilton

"Hamilton was the fourth of nine children born to Sarah Hutton (1780–1817) and Archibald Hamilton (1778–1819),[1] who lived in Dublin at 38 Dominick Street."

https://en.wikipedia.org/wiki/William_Rowan_Hamilton#Early_life

Apollonius of Perga

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"Ferdinand and Madeleine had seven children, out of which Charles was born sixth. He had four brother and two sisters. "

https://www.thefamouspeople.com/profiles/charles-hermite-493.php

Pierre-Simon Laplace

https://www.geni.com/people/Pierre-Simon-de-Laplace/6000000017137824406

Carl Ludwig Siegel

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"Carl Ludwig Siegel was born on December 31, 1896 in Berlin, Germany. His father, whose name remains unknown, was a postal worker. Nothing else is known about his family background or his early childhood."

https://www.thefamouspeople.com/profiles/carl-ludwig-siegel-540.php

Diophantus of Alexandria

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Little is known about the life of Diophantus. He lived in Alexandria, Egypt, during the Roman era, probably from between AD 200 and 214 to 284 or 298. Diophantus has variously been described by historians as either Greek,[2][3][4] non-Greek,[5] Hellenized Egyptian,[6] Hellenized Babylonian,[7] Jewish, or Chaldean.[8] Much of our knowledge of the life of Diophantus is derived from a 5th-century Greek anthology of number games and puzzles created by Metrodorus. One of the problems (sometimes called his epitaph) states:

Muhammed al-Khowârizmi

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"Few details of al-Khwārizmī's life are known with certainty. He was born into a Persian[5] family and Ibn al-Nadim gives his birthplace as Khwarezm[22] in Greater Khorasan (modern Khiva, Xorazm Region, Uzbekistan)."

https://en.wikipedia.org/wiki/Muhammad_ibn_Musa_al-Khwarizmi

Richard Dedekind

Dedekind had three older siblings. As an adult, he never used the names Julius Wilhelm.
"Richard Was the youngest of the four children in the Dedekind family and had an elder sister named Julia with whom he lived for the major part of his life. Just like Richard would, she also remained unmarried throughout her life."

https://www.wikiwand.com/en/Richard_Dedekind

Rudolf and Marianne Gödel had two children, both boys. The elder they named Rudolf after his father, and the younger was Kurt.

http://www-history.mcs.st-andrews.ac.uk/Biographies/Godel.html

Bháscara (II) Áchárya

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Felix Christian Klein

https://www.geni.com/people/Felix-Klein/6000000000300764882

Pascal had two sisters, the younger Jacqueline and the elder Gilberte.

...that "his mother, during those rare minutes when she was free from taking care of the children and the house, was working with a spinning-wheel". Élie had an elder sister Jeanne-Marie (1867–1931) who became a dressmaker; a younger brother Léon (1872–1956) who became a blacksmith working in his father's smithy; and a younger sister Anna (1878–1923), who, partly under Élie's influence, entered École Normale Supérieure (as Élie had before) and chose the career as a mathematics teacher at lycée (secondary school)."

https://en.wikipedia.org/wiki/%C3%89lie_Cartan

https://www.geni.com/people/%C3%89 lie-Cartan/6000000000412695480

Archytas of Tarentum

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"Both Isaac and Sophia came from poor families and therefore, in spite of being highly intelligent with considerable mathematical skills, they could not get university education. The couple had two children; Harold, their first born and Gertrude, two years his younger. The siblings were very close to each other."

https://www.thefamouspeople.com/profiles/g-h-hardy-141.php

Alhazen ibn al-Haytham

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Jean le Rond d'Alembert

"Jean Le Rond d'Alembert was born in Paris, on 16 November 1717, to an author Claudine Guérin de Tencin and an artillery officer Louis-Camus Destouches. He was their illegitimate child, for which his mother abandoned him on the steps of the Parisian church of Saint-Jean-le-Rond, a couple of days after his birth. It was there he received his Christian name, after the name of the patron saint of the church. He was then placed in an orphanage. After a brief stay at the orphanage he was adopted by a glazier’s wife, upon Destouches recommendation. Though de Tencin never accepted d’Alembert as her son, Destouches did not abandon his son and in fact, even paid for d’Alembert’s education. Destouches died in 1726, leaving an annual allowance of 1200 French livres for d’Alembert. It was because of the influence of the Destouches that d’Alembert could enter a prestigious Jansenist school at the age of twelve."

https://www.thefamouspeople.com/profiles/jean-le-rond-dalembert-498.php

Hipparchus of Nicaea

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Andrey N. Kolmogorov

"Andrey Kolmogorov was born in Tambov, about 500 kilometers south-southeast of Moscow, in 1903. His unmarried mother, Maria Y. Kolmogorova, died giving birth to him.[8] Andrey was raised by two of his aunts in Tunoshna (near Yaroslavl) at the estate of his grandfather, a well-to-do nobleman."

Geni gives him a sister with a huge range birth of 1876 to 1934. Also this doesn't square with the unmarried mother-who-died-in-childbirth thing.

https://en.wikipedia.org/wiki/Andrey_Kolmogorov,
https://www.geni.com/people/Andrey-Kolmogorov/6000000040485537149,

https://www.geni.com/list?focus_id=6000000000152401497&group=immediate_family

Eudoxus of Cnidus

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F. Gotthold Eisenstein

"Eisenstein suffered all his life from bad health but at least he survived childhood which none of his five brothers and sisters succeeded in doing. All of them died of meningitis, and Gotthold himself also contracted the disease but he survived it. "

"Eisenstein’s family converted to Protestantism from Judaism just before his birth. He was the oldest of six children and the only one of them to survive childhood meningitis. "

http://www-history.mcs.st-and.ac.uk/Biographies/Eisenstein.html,

Eisenstein’s family converted to Protestantism from Judaism just before his birth. He was the oldest of six children and the only one of them to survive childhood meningitis.

"Jacob had two younger brothers, Nicolaus and Johann Bernoulli. While Nicolaus grew up to be a painter and alderman of Basel, Johann became a mathematician and earned fame for his contributions to infinitesimal calculus." I had to look up a birthdate for Nicolaus.

Geni lists a third brother, Emanual, also younger.

It looks like he was much older than his brothers (more than 10 years)

"https://www.thefamouspeople.com/profiles/jacob-bernoulli-543.php",
https://www.geni.com/people/Jacob-Bernoulli/6000000000161772324,
https://frbr.reasonablegraph.org/archive/item/589?dv=1

https://www.geni.com/list?focus_id=6000000000668688183&group=immediate_family,
https://www.google.com/search?q=johannes+kepler+siblings&oq=Johannes+Kepler+s&aqs=chrome.1.69i57j0l5.1848j0j9&sourceid=chrome&ie=UTF-8

"He was the first-born son of Joseph and Antonina who were poor farmers. He had four younger brothers and sisters."

http://kielich.amu.edu.pl/Stefan_Banach/e-greczek.html

Jacques Hadamard

https://gw.geneanet.org/alanguggenheim?lang=en&n=hadamard&oc=0&p=jacques+salomon

"His parents bought a house in Cuneo but his father continued to work the fields at Tetto Galant with the help of a brother and sister of Giuseppe",

"Giuseppe Peano was the second of the five children of Bartolomeo Peano and Rosa Cavallo. His brother Michele was seven years older. There were two younger brothers, Francesco and Bartolomeo, and a sister, Rosa. Peano’s first home was the farm Tetto Galant, near the village of Spinetta, three miles from Cuneo, the capital of Cuneo province, in Piedmont. When Peano entered school, both he and his brother walked the distance to Cuneo each day. The family later moved to Cuneo so that the children would not have so far to walk. The older brother became a successful surveyor and remained in Cuneo. In 1974 Tetto Galant was still in the possession of the Peano family."

http://www-history.mcs.st-and.ac.uk/Biographies/Peano.html,

https://www.encyclopedia.com/history/historians-and-chronicles/historians-miscellaneous-biographies/giuseppe-peano

Panini of Shalatula

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"The famous philosopher Simone Weil was Weil's only sibling."

https://en.wikipedia.org/wiki/Andr%C3%A9_Weil

"Anna and Niklaus were married on 28 January 1780 and they had eight children. Jakob was the youngest of the children "

http://www-history.mcs.st-and.ac.uk/Biographies/Steiner.html

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https://www.geni.com/people/Gaspard-Monge-comte-de-P%C3%A9luse/6000000021616003967

Hermann G. Grassmann

"Johanne and Justus had twelve children, Hermann being their third child."

http://www-history.mcs.st-and.ac.uk/Biographies/Grassmann.html

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M. E. Camille Jordan

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"Jean-Baptiste Joseph Fourier was born on 21 March 1768 in Auxerre, now the capital of Yonne department, France. His father, a tailor by profession, married his mother after the death of his first wife. Joseph was born ninth of his parents’ twelve children. He also had three siblings from his father’s previous marriage."

https://www.thefamouspeople.com/profiles/joseph-fourier-547.php

Bonaventura Cavalieri

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Jean-Pierre Serre

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Geni has the birth year of the siblings as private

https://www.geni.com/people/Jean-Pierre-Serre-Fields-Medal-1954/4955507715910131307?through=4955483718340042031

Marius Sophus Lie

"Sophus Lie, as he is known, was the sixth and youngest child of a Lutheran pastor, Johann Herman Lie. "

https://www.encyclopedia.com/people/science-and-technology/mathematics-biographies/marius-sophus-lie

https://www.geni.com/people/Albert-Einstein-Nobel-Prize-in-Physics-1921/4481345450320047133

"Galileo was born in Pisa (then part of the Duchy of Florence), Italy, on 15 February 1564,[17] the first of six children of Vincenzo Galilei, a famous lutenist, composer, and music theorist, and Giulia (née Ammannati), who had married in 1562."

https://en.wikipedia.org/wiki/Galileo_Galilei#Early_life_and_family

James C. Maxwell

"His parents had met and married when they were well into their thirties and had lost a baby girl in infancy. He was born when his mother was 40 years old."

https://www.thefamouspeople.com/profiles/james-maxwell-544.php,
https://www.geni.com/people/James-Clerk-Maxwell/6000000004176568650

"After Aristotle’s father died, Proxenus of Atarneus, who was married to Aristotle’s older sister, Arimneste, became Aristotle’s guardian until he came of age."

"Both ‘Arimneste’ and ‘Arimnestus’ translate as ‘Greatly remembered’, and the parallelism of these names suggests that Aristotle may have been the youngest of the three siblings."

https://www.biography.com/people/aristotle-9188415,
https://www.geni.com/people/Aristotle/6000000010654793444,
https://outre-monde.com/2010/08/23/life-of-aristotle/

Girolamo Cardano

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https://www.geni.com/people/Gerolamo-Cardano/6000000043237614146

They had four children, three sons Michael (the eldest and subject of this biography), Patrick Selim (born 5 March 1931, who went on to become an English lawyer and academic) and Joseph (known as Joe, the youngest of the four children who after a mathematics degree from Cambridge University, became a computer scientist working in computer software and telecommunications), and a daughter Selma (who studied English at an American University and lives in America).

https://en.wikipedia.org/wiki/Michael_Atiyah#Education_and_early_life,
https://www.geni.com/people/Sir-Michael-F-Atiyah-Fields-Medal-1966/6000000000491017713,
http://www-history.mcs.st-andrews.ac.uk/Biographies/Atiyah.html

"Anna Selberg married on 30 July 1903 in Halandsdal and they had nine children, five boys and four girls, Atle being the youngest of them. Three of Atle's brothers also became mathematicians. "

http://www-groups.dcs.st-and.ac.uk/history/Biographies/Selberg.html

https://www.geni.com/people/prof-dr-Luitzen-Egbertus-Jan-Brouwer/6000000026179869537

Christiaan Huygens

"His mother was Suzanna van Baerle. She died in 1637, shortly after the birth of Huygens' sister.[10] The couple had five children: Constantijn (1628), Christiaan (1629), Lodewijk (1631), Philips (1632) and Suzanna (1637).[11]"

https://en.wikipedia.org/wiki/Christiaan_Huygens

https://www.thedailybeast.com/alan-turings-brother-he-should-be-alive-today-1

Jean-Victor Poncelet

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https://www.geni.com/people/Victor-Poncelet/344378223710007961,
http://www-groups.dcs.st-and.ac.uk/history/Biographies/Poncelet.html

His mother was Anne-Marie Perrein, but Jean-Victor was an illegitimate child and, although he was born in Metz, he was sent away before he was a year old to be brought up by the Olier family in Saint-Avold, a town to the east of Metz.

Pafnuti Chebyshev

"Lev Pavlovich and Agrafena Ivanova had nine children some of whom followed in their father's military tradition."

http://www-history.mcs.st-andrews.ac.uk/Biographies/Chebyshev.html,
https://www.geni.com/people/%D0%9F%D0%B0%D1%84%D0%BD%D1%83%D1%82%D0%B8%D0%B9-%D0%9B%D1%8C%D0%B2%D0%BE%D0%B2%D0%B8%D1%87-%D0%A7%D0%B5%D0%B1%D1%8B%D1%88%D1%91%D0%B2/6000000016674954857

Henri Léon Lebesgue

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https://www.geni.com/people/Henri-Lebesgue/6000000000327200827

John E. Littlewood

"His father Edward Thornton Littlewood was also a mathematician and was Ninth Wrangler in the Mathematical Tripos at Cambridge in 1882, three years before his eldest son John Edensor was born.
Edward and Sylvia Littlewood went on to have three sons, their second being Martin Wentworth Littlewood, who went on to study medicine, and a third son who tragically died when he was eight years old by falling into a lake from a bridge. "

http://www-history.mcs.st-andrews.ac.uk/Search/historysearch.cgi?SUGGESTION=John+E+Littlewood&CONTEXT=1

F. L. Gottlob Frege

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Had a brother with an estimated birth of 1820 to 1876.

"The German writer Arnold Frege, born in Wismar in 1852, may have been Frege's younger brother, but this has not been confirmed."

https://www.geni.com/people/Gottlob-Frege/6000000027702338493,
https://www.iep.utm.edu/frege/

Alfred had a brother Wacław Teitelbaum to whom he was close as they were growing up.

http://www-history.mcs.st-and.ac.uk/Biographies/Tarski.html,
https://www.geni.com/people/Waclaw-Teitelbaum/6000000039211847304

Shiing-Shen Chern

https://www.geni.com/people/Shiing-Shen-Chern-%E9%99%B3%E7%9C%81%E8%BA%AB/6000000000507179717

James J. Sylvester

"The reason was that his eldest brother decided at this time to emigrate to the United States and he was required to have at least three names before he was allowed to gain residence."

http://www-history.mcs.st-and.ac.uk/Biographies/Sylvester.html

There is a sister listed, but she doesn't have a birth year.

https://www.geni.com/people/prof-dr-Johann-I-Bernoulli/6000000000161333414

https://www.geni.com/list?focus_id=6000000000118952452&group=immediate_family

Johann H. Lambert

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"When Jakab Pólya died in 1897 he left a wife, Anna aged 44 at the time, and five children. George had an older brother Jenö, who was 21 years old and studying medicine when his father died, two older sisters Ilona (10 years older than George) and Flóra (8 years older than George) who went to work for the insurance company Assicurazioni Generali to help support the family, and a younger brother Lásló (4 years younger than George). It is worth pointing out that Jenö, who loved mathematics and always regretted not having pursued that subject, is perhaps as well known to medical people as George is to mathematicians. However, it was Lásló who was considered the brightest of the children, but sadly he was killed in World War I before making a name for himself."

http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Polya.html

https://www.geni.com/people/Felix-Hausdorff/6000000000601600393

Siméon-Denis Poisson

Siméon-Denis was not the first of his parents children but several of his older brothers and sisters had failed to survive. Indeed his health was also very fragile as a child and he was fortunate to pull through. This may have been because his mother, fearing that her young child would die, entrusted him to the care of a nurse to bring him through the critical period. His father had a large influence on his young son, devoting time to teach him to read and write.

http://www-history.mcs.st-andrews.ac.uk/Biographies/Poisson.html,
https://gw.geneanet.org/gntstarpoissons?lang=en&n=poisson&oc=0&p=simeon

Hermann Minkowski

Hermann Minkowski's parents were Lewin Minkowski, a businessman, and Rachel Taubmann. Hermann was his parents' third son. Hermann's oldest brother Max (1844-1930) took over the family business, but he was also an art collector and the French consul in Königsberg. The second brother Oskar (1858-1931) was a physician, best known for his work on diabetes, and father of astrophysicist Rudolph Minkowski (1895-1976). Apart from Max and Oskar, Minkowski also had an older sister, Fanny (1863-1954) and a younger brother, Toby (1873-1906).

http://www-history.mcs.st-andrews.ac.uk/Biographies/Minkowski.html

George D. Birkhoff

https://www.geni.com/people/George-Birkhoff/6000000000481767506

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Little is known about his family background. From their title ‘Khayyami’ it has been assumed that he was born into a family of Muslim tentmakers. It is, however, not known if his father, Ibrahim Khayyami, pursued the family trade.
While many assume that Ibrahim was indeed a tentmaker, some other biographers believe that he was an established physician. However, it is certain that he was aware of the importance of education and was religiously very unorthodox. Nothing is known about Omar’s mother.

https://www.thefamouspeople.com/profiles/omar-khayyam-254.php

Adrien M. Legendre

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It is not surprising that, given these views of Legendre, there are few details of his early life. We have given his place of birth as Paris, as given in [1] and [2], but there is some evidence to suggest that he was born in Toulouse and the family moved to Paris when he was very young. He certainly came from a wealthy family and he was given a top quality education in mathematics and physics at the Collège Mazarin in Paris.

http://www-history.mcs.st-and.ac.uk/Biographies/Legendre.html