A290044 a(n) = |Sum_{k=0..n} (-1)^k*(2k)!!|.

1, 1, 7, 41, 343, 3497, 42583, 602537, 9719383, 176075177, 3539816023, 78209790377, 1883780763223, 49127973630377, 1379201149390423, 41470672541233577, 329725285558734423, 45290937289840177577, 1633052915424520654423, 62144013487721190961577

Offset: 0

Author

XU Pingya, Jul 19 2017

Keywords

• easy
• nonn

Comments

a(20) = 19 * 47 * 73 * 54470291 * 700938921577. Thus a(n) are multiples of 19 for n >= 20.

Crossrefs

Cf. A006882, A138525.

Programs

Mathematica

Table[Abs@ Sum[(-1)^k*(2 k)!!, {k, 0, n}], {n, 0, 19}] (* Michael De Vlieger, Jul 19 2017 *)