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.

Showing 1-2 of 2 results.

A071525 Numbers n such that sigma(phi(n)) = reversal(n).

Original entry on oeis.org

1, 3, 304, 918, 23922, 2388063, 88423222968, 655516144546
Offset: 1

Views

Author

Joseph L. Pe, Jun 02 2002

Keywords

Comments

There are no other terms up to 10^7. - Jason Earls, Jun 04 2002
There are no other terms up to 45*10^7. - Farideh Firoozbakht, Dec 26 2007
There are no other terms up to 10^10. - Donovan Johnson, Oct 24 2013
a(9) > 10^12. - Giovanni Resta, Nov 10 2017

Examples

			sigma(phi(304)) = 403 = reverse(304).

Crossrefs

Cf. A000010, A000203, A135238.

Programs

  • Mathematica
    Select[Range[10^5], DivisorSigma[1, EulerPhi[ # ]] == FromDigits[Reverse[IntegerDigits[ # ]]] &]

Extensions

More terms from Jason Earls, Jun 04 2002
Definition restored by Farideh Firoozbakht, Dec 26 2007
a(7)-a(8) from Giovanni Resta, Nov 10 2017

A135240 Numbers n such that phi(sigma(n))=2n.

Original entry on oeis.org

36, 20575296
Offset: 1

Views

Author

Farideh Firoozbakht, Dec 26 2007

Keywords

Comments

No other terms below 10^10. - Max Alekseyev, Sep 25 2009
6.5*10^12 < a(3) <= 180577942272000000. We also have phi(sigma(n)) = 3*n for n = 144982963520227123200 and phi(sigma(n)) = 4*n for n = 16310583396025551360000 thus these may not be the smallest such n. - Giovanni Resta, Sep 13 2018

Examples

			phi(sigma(20575296))=phi(71238999)=41150592=2*20575296, so 20575296 is in the sequence.

Crossrefs

Cf. A071525, A135238, A135239.

Programs

  • Mathematica
    Do[If[EulerPhi[DivisorSigma[1,n]]==2n,Print[n]],{n,350000000}]
Showing 1-2 of 2 results.

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