A120390 Sum of digits of double factorial numbers.

1, 2, 3, 8, 6, 12, 6, 15, 18, 15, 18, 18, 18, 18, 18, 18, 36, 45, 45, 36, 45, 45, 36, 54, 63, 45, 72, 45, 72, 72, 90, 90, 90, 81, 108, 81, 108, 81, 126, 90, 108, 108, 144, 81, 144, 90, 135, 117, 144, 126, 153, 108, 180, 135, 180, 135, 171, 180, 180, 171, 198, 180, 198

Offset: 1

Author

Paolo P. Lava and Giorgio Balzarotti, Jun 30 2006

Keywords

• easy
• nonn
• base

Comments

If n > 10, then 9 divides a(n). - Michel Lagneau, Dec 22 2011

Examples

			5!! = 5*3*1 = 15 --> 1+5 = 6
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A004152, A006882, A007953.

Programs

Maple

P:=proc(n) local i,j,k,t1,t2; for i from 1 by 1 to n do j:=i; k:=i-2; while k>0 do j:=j*k; k:=k-2; od; t1:=j; t2:=0; while t1 <> 0 do t2:= t2+(t1 mod 10); t1 := floor(t1/10); od; print(t2); od; end: P(100);

Mathematica

Table[Total[IntegerDigits[n!!]],{n,70}] (* Harvey P. Dale, May 19 2021 *)

Formula

a(n) = A007953(A006882(n)) - Eric Chen, Jun 13 2018