|
%I #13 Oct 25 2021 07:16:34
%S 0,1,20,255,3064,34743,380939,4089499,43282317,453472867,4715695283,
%T 48760330737,501941505404,5148657883067,52659616820819
%N Number of positive integers with n digits that are the product of two integers ending with 7.
%C a(n) is the number of n-digit numbers in A348054.
%F a(n) < A052268(n).
%F Conjecture: lim_{n->infinity} a(n)/a(n-1) = 10.
%o (Python)
%o def a(n):
%o lo, hi = 10**(n-1), 10**n
%o return len(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))
%o print([a(n) for n in range(1, 9)]) # _Michael S. Branicky_, Sep 26 2021
%Y Cf. A017377, A052268, A348054.
%Y Cf. A346509 (ending with 1), A346629 (ending with 2), A346952 (ending with 3), A347255 (ending with 4), A337855 (ending with 5), A337856 (ending with 6), A348549 (ending with 8).
%K nonn,base,hard,more
%O 1,3
%A _Stefano Spezia_, Sep 26 2021
%E a(9)-a(11) from _Michael S. Branicky_, Sep 26 2021
%E a(12)-a(15) from _Martin Ehrenstein_, Oct 25 2021
|
