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 A067004 Number of numbers <= n with same number of divisors as n. 11
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS Paul Tek, Table of n, a(n) for n = 1..10000 FORMULA Ordinal transform of A000005. - Franklin T. Adams-Watters, Aug 28 2006 a(A000040(n)^(p-1)) = n if p is prime. - Robert Israel, May 04 2015 EXAMPLE a(10)=3 since 6,8,10 each have four divisors. a(11)=5 since 2,3,5,7,11 each have two divisors. MAPLE 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: seq(A[n], n=1..N); # Robert Israel, May 04 2015 PROG (PARI) a(n)=my(d=numdiv(n)); sum(k=1, n, numdiv(k)==d) \\ Charles R Greathouse IV, Sep 02 2015 CROSSREFS 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. Cf. also A079788, A138009. A047983(n) = a(n)-1. Sequence in context: A161904 A323638 A249336 * A117920 A079617 A079616 Adjacent sequences:  A067001 A067002 A067003 * A067005 A067006 A067007 KEYWORD nonn AUTHOR Henry Bottomley, Dec 21 2001 STATUS approved

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Last modified February 19 09:33 EST 2020. Contains 332041 sequences. (Running on oeis4.)