OFFSET
1,1
COMMENTS
a(n) is even.
Even numbers n such that n^2+1 divides (n/2)!. - Robert Israel, Jun 19 2019
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
70 is in the sequence because 70^2+1 = 4901 = 29*13^2 divides 70!!= 355044260642859198243475901411974413130137600000000 =
2 ^ 67 *3 ^ 15
*5 ^ 8 * 7 ^ 5 * 11 ^ 3 * 13 ^ 2 * 17 ^ 2 *19 * 23 * 29* 31
MAPLE
with(numtheory):for n from 1 to 1300 do:x:=irem(doublefactorial(n), n^2+1): if x=0 then printf(`%d, `, n):else fi:od:
MATHEMATICA
Select[Range[1200], Divisible[#!!, #^2+1]&] (* Harvey P. Dale, Oct 14 2015 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Michel Lagneau, Aug 03 2012
STATUS
approved