OFFSET
1,2
COMMENTS
a(n) is divisible by 107 for n >= 53. - Robert Israel, Dec 01 2015
Last digit is 7 for n > 1. Therefore there is no square in this sequence except 1. - Altug Alkan, Dec 01 2015
a(n) is the rank of [2,1, 4,3, ..., 2n,2n-1] within the permutations of [1, 2, ... 2n-1, 2n] in lexicographic order. See A375302 for the ranking function. - Hugo Pfoertner, Aug 25 2024
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..225
FORMULA
Recurrence: a(1) = 1, a(2) = 7, a(n) = (4*n^2-6*n+3)*a(n-1) - 2*(n-1)*(2*n-1)*a(n-2). - Vladimir Reshetnikov, Oct 28 2015
MAPLE
a:=proc(n) options operator, arrow: sum(factorial(2*k-1), k=1..n) end proc: seq(a(n), n=1..14); # Emeric Deutsch, Mar 31 2008
MATHEMATICA
Table[Sum[(2i - 1)!, {i, n}], {n, 15}] (* Stefan Steinerberger, Mar 25 2008 *)
PROG
(PARI) a(n) = sum(k=1, n, (2*k-1)!); \\ Michel Marcus, Oct 28 2015
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Leroy Quet, Mar 23 2008
EXTENSIONS
STATUS
approved