The Counting Thread

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636 = 22 × 3 × 53, sum of ten consecutive primes (43 + 47 + 53 + 59 + 61 + 67 + 71 + 73 + 79 + 83), Smith number, Mertens function(636) = 0

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637 = 72 × 13, Mertens function(637) = 0, decagonal number

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638 = 2 × 11 × 29, sphenic number, sum of four consecutive primes (151 + 157 + 163 + 167), nontotient, centered heptagonal number

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639

BrilloPad 244
miffy 178
AndyGarbs 69
Pinto 27
TheFaQQer 21

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640 = 27 × 5, Harshad number

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641 prime number, Sophie Germain prime, factor of 4294967297 (the smallest nonprime Fermat number), Chen prime, Eisenstein prime with no imaginary part, Proth prime

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642 = 2 × 3 × 107, sphenic number

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643 prime number, largest prime factor of 123456

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644 = 22 × 7 × 23, nontotient, Harshad number. Common umask.

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645 = 3 × 5 × 43, sphenic number, Smith number, Harshad number