A109657 Numbers n such that the sum of the digits of Sum_{k=1..n} (k!) is divisible by n.

1, 3, 6, 9, 12, 18, 54, 117, 272, 294, 296, 320, 783, 1125, 2088, 3375, 16164, 16407, 26286, 26777, 26784, 27516, 27568, 45945, 74970, 124236, 125589, 208116, 348705, 583746, 586218, 586353, 586368, 586536, 588567, 2712944, 2714655, 2714912, 2720288, 2720399

Offset

1,2

Comments

Most, but not all, of the terms in this sequence are divisible by 3; is this a coincidence?

In general, terms should be more likely to occur in regions where the number of digits in the sum of the first n factorials is close to an integer multiple of 2*n/9. This happens, e.g., around n = 268, 449, 752, 1257, 2100, 3506, 5851, 9763, 16290, 27177, 45337, 75631, 126165, etc. - Jon E. Schoenfield, Jun 16 2010

Numbers n such that A349403(n) (mod n) == 0. - Kevin P. Thompson, Nov 28 2021

a(43) > 5570000. - Kevin P. Thompson, Nov 28 2021

Links

Kevin P. Thompson, Table of n, a(n) for n = 1..42

Example

6 is a member of the sequence since Sum_{k=1..6}(k!) = 1! + 2! + 3! + 4! + 5! + 6! = 1 + 2 + 6 + 24 + 120 + 720 = 873 which has a digit sum of 18 that is divisible by 6.

Mathematica

s = 0; Do[s += n!; k = Plus @@ IntegerDigits[s]; If[Mod[k, n] == 0, Print[n]], {n, 1, 10000}]
Module[{nn=2721000, sf}, sf=Total[IntegerDigits[#]]&/@Accumulate[Range[nn]!]; Select[ Thread[ {Range[nn], sf}], Mod[#[[2]], #[[1]]]==0&]][[;; , 1]] (* Harvey P. Dale, Apr 16 2023 *)

Crossrefs

Cf. A000142, A007489, A349403.

Sequence in context: A232920 A092421 A375026 * A175589 A348339 A282759

Adjacent sequences: A109654 A109655 A109656 * A109658 A109659 A109660

Keyword

nonn,base

Author

Ryan Propper, Aug 06 2005

Extensions

More terms from Jon E. Schoenfield, Jun 16 2010

a(26)-a(40) from Kevin P. Thompson, Nov 28 2021

Status

approved