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A067004
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Number of numbers <= n with same number of divisors as n.
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13
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1, 1, 2, 1, 3, 1, 4, 2, 2, 3, 5, 1, 6, 4, 5, 1, 7, 2, 8, 3, 6, 7, 9, 1, 3, 8, 9, 4, 10, 2, 11, 5, 10, 11, 12, 1, 12, 13, 14, 3, 13, 4, 14, 6, 7, 15, 15, 1, 4, 8, 16, 9, 16, 5, 17, 6, 18, 19, 17, 1, 18, 20, 10, 1, 21, 7, 19, 11, 22, 8, 20, 2, 21, 23, 12, 13, 24, 9, 22, 2, 2, 25, 23, 3, 26, 27
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OFFSET
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1,3
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LINKS
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FORMULA
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EXAMPLE
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a(10)=3 since 6,8,10 each have four divisors. a(11)=5 since 2,3,5,7,11 each have two divisors.
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MAPLE
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N:= 1000: # to get a(1) to a(N)
R:= Vector(N):
for n from 1 to N do
v:= numtheory:-tau(n);
R[v]:= R[v]+1;
A[n]:= R[v];
od:
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MATHEMATICA
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b[_] = 0;
a[n_] := a[n] = With[{t = DivisorSigma[0, n]}, b[t] = b[t]+1];
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PROG
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CROSSREFS
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Cf. A000005, A008479, A058933, A067003. Inverse of A000040, A001248, A030513, A030514, A030515, A030516, A030626, A030627, A030628, A030629, A030630, A030631, A030632, A030633, A030634, A030635, A030636, A030637, A030638 etc.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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