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A050472 Numbers m such that 2*phi(m) = phi(m+1). 9
2, 4, 16, 154, 256, 286, 364, 804, 1066, 2146, 3382, 4550, 6106, 7700, 8176, 9268, 11284, 12556, 12970, 16402, 19228, 19276, 20272, 25132, 26404, 27346, 29154, 29574, 35644, 36418, 38368, 39646, 40494, 47214, 52234, 54652, 65536, 84862 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

REFERENCES

R. K. Guy, Unsolved Problems Number Theory, Sect. B36.

LINKS

Giovanni Resta, Table of n, a(n) for n = 1..10000 (first 1000 terms from Donovan Johnson)

FORMULA

Conjecture : a(n)/n^3 is bounded. Does lim n -> infinity a(n)/n^3 = 2 ? - Benoit Cloitre, Aug 07 2002

a(n) = A171271(n) - 1. - Ray Chandler, May 01 2015

EXAMPLE

phi(256)=128, phi(256+1)=2*128, so 256 is a member of the sequence.

MAPLE

select(n->phi(n+1)=2*phi(n), [$1..10^5]); # Paolo P. Lava, Aug 02 2018

PROG

(PARI) isok(n) = 2*eulerphi(n) == eulerphi(n+1); \\ Michel Marcus, Aug 02 2018

CROSSREFS

Cf. A000010, A001274, A050473, A171271.

Sequence in context: A073924 A061588 A202360 * A109457 A105788 A217727

Adjacent sequences:  A050469 A050470 A050471 * A050473 A050474 A050475

KEYWORD

nonn

AUTHOR

Jud McCranie, Dec 24 1999

STATUS

approved

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Last modified April 25 21:06 EDT 2019. Contains 322461 sequences. (Running on oeis4.)