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Nawal A. Hady
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Christian GoldbachBorn 18 Mar 1690; died 20 Nov 1764 at age 74.Russian mathematician whose contributions to number theory include Goldbach's conjecture, formulated in a letter to Leonhard Euler dated 7 Jul 1742. Stated in modern terms it proposes that: "Every even natural number greater than 2 is equal to the sum of two prime numbers." It has been checked by computer for vast numbers  up to at least 4 x 1014  but still remains unproved. Goldbach made another conjecture that every odd number is the sum of three primes, on which Vinogradov made progress in 1937. (It has been checked by computer for vast numbers, but remains unproved.) Goldbach also studied infinite sums, the theory of curves and the theory of equations.«[Image: Letter to Euler, in which Goldbach presented his conjecture.] 
Hair pulling
A six year old comes crying to his Mother because his little sister pulled his hair.
"Don't be angry," the Mother says, "Your little sister doesn't realize that pulling hair hurts."
A short while later, there's more crying, and the Mother goes to investigate.
This time the sister is bawling and her brother says...
"Now she knows."