login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A300779 Odd numbers x such that x and x + 2 are both sums of divisors, i.e., elements of A000203. 1
1, 13, 91, 241573, 38152387, 139415801707, 55342019130181, 61166380109329, 417542026135897, 417542026135897, 13805828672331787 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

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.

LINKS

Table of n, a(n) for n=1..11.

Seqfan Mailing List Thread, Consecutive sigmas, with contributions from Franklin T. Adams-Watters, Hugo Pfoertner and Emmanuel Vantieghem.

EXAMPLE

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).

CROSSREFS

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

Sequence in context: A022578 A090020 A092469 * A275918 A027670 A055608

Adjacent sequences:  A300776 A300777 A300778 * A300780 A300781 A300782

KEYWORD

nonn,hard,more

AUTHOR

Hugo Pfoertner, Mar 12 2018

EXTENSIONS

a(5) from Emmanuel Vantieghem

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

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 12 04:27 EDT 2021. Contains 343810 sequences. (Running on oeis4.)