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A341434 a(n) is the number of bases 1 < b < n in which n is divisible by its product of digits. 2
0, 0, 1, 1, 1, 2, 2, 3, 2, 2, 1, 5, 2, 3, 4, 6, 1, 5, 1, 5, 4, 4, 1, 9, 2, 2, 4, 5, 1, 7, 3, 9, 4, 2, 3, 12, 1, 2, 3, 10, 1, 7, 2, 7, 7, 2, 1, 15, 2, 5, 3, 6, 1, 10, 3, 10, 4, 3, 1, 14, 1, 2, 7, 14, 3, 8, 1, 6, 3, 6, 1, 20, 2, 3, 8, 7, 3, 7, 1, 16, 7, 2, 1, 14 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,6

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

FORMULA

a(n) > 0 for all numbers n > 2 since n in base b = n-1 is 11.

a(n) > 1 for all even numbers > 4 since n in base b = n-2 is 12. Similarly, a(n) > 1 for all composite numbers > 4 since if n = k*m, then n is divisible by its product of digits in bases n-m and n-k.

a(p) > 1 for primes p in A085104.

a(p) > 2 for primes p in A119598 (i.e., 31, 8191, ...).

a(n) >= A088323(n), with equality if n = 4 or if n is a prime.

EXAMPLE

a(3) = 1 since 3 is divisible by its product of digits only in base 2: 3 = 11_2 and 1*1 | 3.

a(6) = 2 since 6 is divisible by its product of digits in 2 bases: in base 4, 6 = 12_4 and 1*2 | 6, and in base 5, 6 = 11_5 and 1*1 | 6.

MATHEMATICA

q[n_, b_] := (p = Times @@ IntegerDigits[n, b]) > 0 && Divisible[n, p]; a[n_] := Count[Range[2, n], _?(q[n, #] &)]; Array[a, 100]

PROG

(PARI) a(n) = sum(b=2, n-1, my(x=vecprod(digits(n, b))); x && !(n%x)); \\ Michel Marcus, Feb 12 2021

CROSSREFS

Cf. A007602, A068953, A080221, A085104, A088323, A119598.

Sequence in context: A304199 A251140 A259977 * A115312 A237721 A254296

Adjacent sequences:  A341431 A341432 A341433 * A341435 A341436 A341437

KEYWORD

nonn,base

AUTHOR

Amiram Eldar, Feb 11 2021

STATUS

approved

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Last modified June 17 05:08 EDT 2021. Contains 345080 sequences. (Running on oeis4.)