OFFSET
0,3
COMMENTS
The prime indices of a(n) are the signature of n!, which is row n of A115627.
FORMULA
EXAMPLE
We have 7! = 2^4 * 3^2 * 5^1 * 7^1, so a(7) = prime(4)*prime(2)*prime(1)*prime(1) = 84.
The sequence of terms together with their prime indices begins:
1: {}
1: {}
2: {1}
4: {1,1}
10: {1,3}
20: {1,1,3}
42: {1,2,4}
84: {1,1,2,4}
204: {1,1,2,7}
476: {1,1,4,7}
798: {1,2,4,8}
1596: {1,1,2,4,8}
3828: {1,1,2,5,10}
7656: {1,1,1,2,5,10}
12276: {1,1,2,2,5,11}
24180: {1,1,2,3,6,11}
36660: {1,1,2,3,6,15}
73320: {1,1,1,2,3,6,15}
120840: {1,1,1,2,3,8,16}
241680: {1,1,1,1,2,3,8,16}
MATHEMATICA
Table[Times@@Prime/@Last/@If[(n!)==1, {}, FactorInteger[n!]], {n, 0, 30}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, May 08 2019
STATUS
approved