A356860 a(n) is the number of digits in the product of the first n numbers not divisible by 5.

1, 1, 1, 1, 2, 3, 4, 4, 5, 6, 7, 9, 10, 11, 12, 13, 15, 16, 17, 19, 20, 22, 23, 24, 26, 27, 29, 30, 32, 33, 35, 37, 38, 40, 41, 43, 45, 46, 48, 50, 51, 53, 55, 57, 58, 60, 62, 64, 65, 67, 69, 71, 73, 74, 76, 78, 80, 82, 84, 85, 87, 89, 91, 93, 95, 97, 99, 101, 103

Offset: 0

Author

Stefano Spezia, Sep 01 2022

Keywords

• nonn
• base

Links

• Harvey P. Dale, Table of n, a(n) for n = 0..1000

Crossrefs

Cf. A047201, A055642, A356858.

Cf. A356859 (number of zero digits), A356861 (number of nonzero digits).

Programs

Mathematica

Table[Length[IntegerDigits[Product[Floor[(5i-1)/4], {i,n}]]], {n,0,68}]
Join[{1},IntegerLength/@FoldList[Times,Table[If[Mod[n,5]==0,Nothing,n],{n,0,100}]]] (* Harvey P. Dale, Jul 20 2025 *)

Python

from math import prod
def a(n): return len(str(prod((5*k-1)//4 for k in range(1, n+1))))
print([a(n) for n in range(69)]) # Michael S. Branicky, Sep 01 2022

Formula

a(n) = A055642(A356858(n)).