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A348055
Number of positive integers with n digits that are the product of two integers ending with 7.
4
0, 1, 20, 255, 3064, 34743, 380939, 4089499, 43282317, 453472867, 4715695283, 48760330737, 501941505404, 5148657883067, 52659616820819
OFFSET
1,3
COMMENTS
a(n) is the number of n-digit numbers in A348054.
FORMULA
a(n) < A052268(n).
Conjecture: lim_{n->infinity} a(n)/a(n-1) = 10.
PROG
(Python)
def a(n):
lo, hi = 10**(n-1), 10**n
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))
print([a(n) for n in range(1, 9)]) # Michael S. Branicky, Sep 26 2021
CROSSREFS
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).
Sequence in context: A306290 A075469 A028032 * A025986 A028027 A268764
KEYWORD
nonn,base,hard,more
AUTHOR
Stefano Spezia, Sep 26 2021
EXTENSIONS
a(9)-a(11) from Michael S. Branicky, Sep 26 2021
a(12)-a(15) from Martin Ehrenstein, Oct 25 2021
STATUS
approved