OFFSET
1,3
COMMENTS
a(n) is the number of n-digit numbers in A346507.
FORMULA
Conjecture: Lim_{n->infinity} a(n)/a(n-1) = 10.
PROG
(Python)
def A346507upto(lim): return sorted(set(a*b for a in range(11, lim//11+1, 10) for b in range(a, lim//a+1, 10)))
def a(n): return len(A346507upto(10**n)) - len(A346507upto(10**(n-1)))
print([a(n) for n in range(1, 9)]) # Michael S. Branicky, Jul 22 2021
(PARI) a(n) = {my(res = 0); forstep(i = 10^(n-1) + 1, 10^n, 10, f = factor(i); if(bigomega(f) == 1, next); d = divisors(f); for(j = 2, (#d~ + 1)>>1, if(d[j]%10 == 1 && d[#d + 1 - j]%10 == 1, res++; next(2) ) ) ); res } \\ David A. Corneth, Jul 22 2021
CROSSREFS
KEYWORD
nonn,base,hard,more
AUTHOR
Stefano Spezia, Jul 21 2021
EXTENSIONS
a(6)-a(9) from Michael S. Branicky, Jul 22 2021
a(10) from David A. Corneth, Jul 22 2021
a(11) from Michael S. Branicky, Jul 23 2021
a(11) corrected and extended with a(12) by Martin Ehrenstein, Aug 03 2021
a(13)-a(14) from Martin Ehrenstein, Aug 05 2021
STATUS
approved