cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A281685 Numbers k such that A001221(k) = A001221(A003415(k)).

Original entry on oeis.org

4, 16, 18, 20, 21, 24, 26, 27, 33, 35, 38, 40, 44, 45, 48, 50, 51, 52, 56, 57, 62, 63, 65, 68, 69, 74, 75, 76, 77, 80, 85, 86, 91, 92, 93, 95, 96, 98, 99, 104, 106, 108, 111, 115, 117, 119, 122, 123, 126, 129, 133, 134, 135, 136, 141, 143, 144, 145, 146, 147
Offset: 1

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Author

Colin Barker, Jan 27 2017

Keywords

Comments

Numbers k such that the number of distinct prime divisors of k is equal to the number of distinct prime divisors of the arithmetic derivative of k.

Crossrefs

Programs

  • Mathematica
    a[n_] := If[Abs@n < 2, 0, n Total[#2/#1 & @@@ FactorInteger[Abs@n]]];
    Select[Range[100], PrimeNu[#] == PrimeNu[a[#]] &] (* G. C. Greubel, Apr 23 2017 *)
  • PARI
    arde(n) = sum(i=1, #f=factor(n)~, n/f[1, i]*f[2, i])
    select(k->omega(k)==omega(arde(k)), vector(200, k, k+1))