A138586 a(1) = 1; a(n) = a(n-1) + (n!)^7.

1, 129, 280065, 4586751489, 358322666751489, 100306488365546751489, 82606511560391889386751489, 173238283180457843219993066751489, 828593116199250458889895450218986751489

Offset: 1

Author

Jonathan Vos Post, May 18 2008

Keywords

• easy
• nonn

Comments

After a(1) = 1 these are all divisible by 3. a(n)/3 is prime (i.e. a(n) is semiprime) for n = 2, 4 (i.e. (1!)^7 + (2!)^7 + (3!)^7 + (4!)^7 = 4586751489 = 3 * 1528917163) and then when next?

Crossrefs

Cf. A000142, A001015, A104344, A100288.

Formula

a(n) = Sum_{k=1..n} (k!)^7 = Sum_{k=1..n} A001015(A000142(n)).