A348546 Number of positive integers with n digits that are equal both to the product of two integers ending with 3 and to that of two integers ending with 7.
0, 0, 8, 129, 1771, 21802, 252793, 2826973, 30872783
Offset: 1
Programs
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Mathematica
Table[{lo, hi}={10^(n-1), 10^n}; Length@Select[Intersection[Union@Flatten@Table[a*b, {a, 3, Floor[hi/3], 10}, {b, a, Floor[hi/a], 10}], Union@Flatten@Table[a*b, {a, 7, Floor[hi/7], 10}, {b, a, Floor[hi/a], 10}]], lo<# -
Python
def a(n): lo, hi = 10**(n-1), 10**n return len(set(a*b for a in range(3, hi//3+1, 10) for b in range(a, hi//a+1, 10) if lo <= a*b < hi) & set(a*b for a in range(7, hi//7+1, 10) for b in range(a, hi//a+1, 10) if lo <= a*b < hi)) print([a(n) for n in range(1, 9)]) # Michael S. Branicky, Oct 22 2021
Formula
Extensions
a(9) from Michael S. Branicky, Oct 22 2021
Comments