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A347749
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Number of positive integers with n digits and final digit 6 that are equal to the product of two integers ending with the same digit.
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2
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OFFSET
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1,2
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COMMENTS
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a(n) is the number of n-digit numbers in A347747.
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LINKS
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Table of n, a(n) for n=1..10.
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FORMULA
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a(n) < A052268(n).
a(n) = A337856(n) + A347255(n) - A347748(n).
Conjecture: lim_{n->infinity} a(n)/a(n-1) = 10.
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MATHEMATICA
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Table[{lo, hi}={10^(n-1), 10^n}; Length@Select[Union[Union@Flatten@Table[a*b, {a, 4, Floor[hi/4], 10}, {b, a, Floor[hi/a], 10}], Union@Flatten@Table[a*b, {a, 6, Floor[hi/6], 10}, {b, a, Floor[hi/a], 10}]], lo<#<hi&], {n, 8}]
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PROG
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(Python)
def a(n):
lo, hi = 10**(n-1), 10**n
return len(set(a*b for a in range(4, hi//4+1, 10) for b in range(a, hi//a+1, 10) if lo <= a*b < hi) | set(a*b for a in range(6, hi//6+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 06 2021
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CROSSREFS
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Cf. A017341, A052268, A324297, A337856, A347253, A347255, A347747, A347748.
Sequence in context: A093185 A198006 A339654 * A075132 A303919 A208961
Adjacent sequences: A347746 A347747 A347748 * A347750 A347751 A347752
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KEYWORD
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nonn,base,hard,more
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AUTHOR
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Stefano Spezia, Sep 12 2021
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EXTENSIONS
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a(9)-a(10) from Michael S. Branicky, Oct 06 2021
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STATUS
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approved
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