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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A300779 Odd numbers x such that x and x + 2 are both sums of divisors, i.e., elements of A000203.

Original entry on oeis.org

1, 13, 91, 241573, 38152387, 139415801707, 55342019130181, 61166380109329, 417542026135897, 417542026135897, 13805828672331787
Offset: 1

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Author

Hugo Pfoertner, Mar 12 2018

Keywords

Comments

If some x or x + 2 is in A300869, i.e., it has more than one representation as sigma(m), as for x = 417542026135897 = sigma((4*17*209459)^2) = sigma((5*17*209459)^2) = sigma((2*7723267)^2) - 2, then it is listed with multiplicity and all corresponding pairs of numbers are provided in A300780.

Examples

			a(1) = 1 because 1 = sigma(1) and 3 = sigma(2),
a(2) = 13: 13 = sigma(9) and 15 = sigma(8),
a(3) = 91: 91 =sigma(36), 93 = sigma(50),
a(4) = 241573: 241573 = sigma(241081), 241575 = sigma(117128),
a(5) = 38152387: 38152387 = sigma(15069924), 38152389 = sigma(23011209).

Links

  • Seqfan Mailing List Thread, Consecutive sigmas, with contributions from _Franklin T. Adams-Watters_, _Hugo Pfoertner_ and _Emmanuel Vantieghem_.

Crossrefs

Cf. A000203, A002191, A083531, A300780 (numbers corresponding to sigma values), A300869.

Extensions

a(5) from Emmanuel Vantieghem
a(6)-a(11) from Giovanni Resta, Mar 13 2018
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