OFFSET
1,1
COMMENTS
Every odd integer k not ending with 5 has a multiple that is a repunit (see A099679), hence a(n) <= the concatenation of this repunit with this odd number (see example a(33)).
FORMULA
a(10*k) = 20*k.
EXAMPLE
a(16) = 32 because 32 is the smallest proper multiple of 16 such that 1*6 = 3*2.
a(33) = 11111133 is the concatenation of 111111 (that is the smallest repunit multiple of 33) with 33.
MATHEMATICA
prodig[n_] := Times @@ IntegerDigits[n]; a[n_] := Module[{k = 2*n, p = prodig[n]}, While[prodig[k] != p, k += n]; k]; Array[a, 20] (* Amiram Eldar, Jan 15 2021 *)
PROG
(PARI) f(n) = vecprod(digits(n)); \\ A007954
a(n) = my(x = f(n), k = 2); while(f(k*n) != x, k++); k*n; \\ Michel Marcus, Jan 15 2021
(Python)
from math import prod
def pd(n): return prod(map(int, str(n)))
def a(n):
pdn, f = pd(n), 2
while pd(f*n) != pdn: f += 1
return f*n
print([a(n) for n in range(1, 27)]) # Michael S. Branicky, Jan 16 2021
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Bernard Schott, Jan 15 2021
EXTENSIONS
More terms from Amiram Eldar, Jan 15 2021
STATUS
approved