640

640 = 27 × 5, Harshad number, refactorable number, hexadecagonal number, number of 1's in all partitions of 24 into odd parts, number of acres in a square mile 641 = prime number, Sophie Germain prime,factor of 4294967297 (the smallest nonprime Fermat number), Chen prime, Eisenstein prime with no imaginary part, Proth prime