A012800 Expansion of e.g.f. sech(sec(x)*tan(x)) (only even powers).

1, -1, -15, -177, 8225, 1259231, 82092945, -6743602257, -3731314041535, -658546620230977, 68462273756819505, 102601344100683911439, 37004621343746850934625, -3605666448860960629195873, -14335634030229955250450088495, -9176884449766238892958778771601, 311159935826841412749460317933185

Offset

0,3

Links

Table of n, a(n) for n=0..16.

Example

E.g.f. = 1 - 1/2!*x^2 - 15/4!*x^4 - 177/6!*x^6 + 8225/8!*x^8+ ...

Mathematica

With[{nn=30}, Take[CoefficientList[Series[Sech[Sec[x]Tan[x]], {x, 0, nn}], x] Range[0, nn]!, {1, -1, 2}]] (* Harvey P. Dale, Jul 02 2017 *)

Crossrefs

Sequence in context: A081043 A155000 A009146 * A016216 A244604 A001717

Adjacent sequences: A012797 A012798 A012799 * A012801 A012802 A012803

Keyword

sign

Author

Patrick Demichel

Extensions

a(14)-a(16) from Stefano Spezia, Feb 24 2026

Status

approved