OFFSET
1,1
COMMENTS
A term cannot be a multiple of either 2 or 5 since removing these factors does not alter the number of divisors ending with 3 or 7.
All terms must be in A331029.
EXAMPLE
a(1) = 3: divisors of 3 are 1, 3*. (* marks divisors ending in 3 or 7.)
a(2) = 21: divisors of 21 are 1, 3*, 7*, 21.
a(3) = 63: divisors of 63 are 1, 3*, 7*, 9, 21, 63*.
a(4) = 189: divisors of 189 are 1, 3*, 7*, 9, 21, 27*, 63*, 189.
PROG
(PARI) a(n)={forstep(k=1, oo, 2, if(sumdiv(k, d, abs(d%10-5)==2) == n, return(k)))}
(PARI) \\ faster program: needs lista331029 defined in A331029.
a(n)={my(lim=1); while(1, lim*=10; my(S=lista331029(lim)); for(i=1, #S, my(k=S[i]); if(sumdiv(k, d, abs(d%10-5)==2)==n, return(k))))}
(Python)
from sympy import divisors
def count37(iterable): return sum(i%10 in [3, 7] for i in iterable)
def a(n):
m = 2
while count37(divisors(m)) != n: m += 1
return m
print([a(n) for n in range(1, 17)]) # Michael S. Branicky, Mar 21 2021
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Andrew Howroyd, Jan 08 2020
STATUS
approved