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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A232720 Numbers such that 2*phi(n-2) = n-1.

Original entry on oeis.org

3, 5, 17, 257, 65537, 83623937, 4294967297, 6992962672132097
Offset: 1

Views

Author

Michel Marcus, Nov 28 2013

Keywords

Comments

Numbers such that A074057(n) = 0.
Note that the first 5 terms are Fermat primes (A019434), while a(6) is prime but is not a Fermat number, and a(7) is (F5 from A000215) but is not prime. Note also that F6, F7, F8 and F9 do not belong to this sequence.

Crossrefs

Cf. A000010, A074057.

Programs

  • PARI
    is_A232720(n) = 2*eulerphi(n-2) == n-1

Formula

a(n) = A050474(n) + 2. - Jaroslav Krizek, Feb 25 2015

Extensions

a(8) from Jaroslav Krizek, Feb 25 2015
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Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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