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.

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A341933 a(n) = A023896(n) mod A000203(n).

Original entry on oeis.org

0, 1, 3, 4, 4, 6, 5, 1, 1, 2, 7, 24, 8, 18, 12, 2, 10, 15, 11, 38, 30, 2, 13, 36, 2, 30, 3, 0, 16, 48, 17, 4, 42, 2, 36, 34, 20, 42, 20, 50, 22, 60, 23, 20, 72, 2, 25, 12, 3, 35, 24, 36, 28, 6, 20, 72, 66, 2, 31, 144, 32, 66, 94, 8, 48, 84, 35, 80, 78, 120, 37, 84, 38, 78, 12, 108, 6, 96, 41, 164
Offset: 1

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Author

J. M. Bergot and Robert Israel, Feb 23 2021

Keywords

Comments

a(k) is the sum of totatives of k modulo the sum of divisors of k.
If p is an odd prime, a(p) = (p+3)/2 and a(p^2) = (p-1)/2.
If p is a prime == 5 (mod 6), a(2*p) = 2.
If p is a prime == 1 (mod 6), a(2*p) = 2*p+4.
Are 2, 8 and 9 the only solutions to a(k) = 1?

Examples

			a(6) = 6 because the sum of totatives of 6 is 1+5 = 6, the sum of divisors of 6 is 1+2+3+6 = 12, and 6 mod 12 = 6.
		

Crossrefs

Programs

  • Maple
    f:= n -> n*numtheory:-phi(n)/2 mod numtheory:-sigma(n):
    map(f, [$1..100]);
  • Mathematica
    Array[Mod[# EulerPhi[#]/2 + Boole[# == 1]/2, DivisorSigma[1, #]] &, 80] (* Michael De Vlieger, Feb 23 2021 *)
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