OFFSET
0,1
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..100
FORMULA
a(2n) == 21 (mod 100), n>0.
a(n) = (2n)! * [x^(2n)] sec(x) + sec(x)^2. - Ira M. Gessel, Apr 18 2013
MAPLE
A000111 := proc(n) if n = 0 then 1; else 2^n*abs(euler(n, 1/2)+euler(n, 1) ); end if; end proc:
MATHEMATICA
terms = 20;
CoefficientList[Sec[x] + Sec[x]^2 + O[x]^(2 terms - 1), x^2] Range[0, 2 terms - 2, 2]! (* Jean-François Alcover, Nov 19 2020 *)
PROG
(Python)
from itertools import accumulate, islice
def A141458_gen(): # generator of terms
yield 2
blist = (0, 1)
while True:
yield (blist := tuple(accumulate(reversed(blist), initial=0)))[-1] + (blist := tuple(accumulate(reversed(blist), initial=0)))[-1]
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul Curtz, Aug 08 2008
STATUS
approved