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A330251 Numbers k such that phi(k) = phi(k+3), where phi (A000010) is Euler's totient function. 1
3, 5, 8720288051472, 9134280520365, 41544070492925, 42466684755492, 51363581614342, 68616494581632, 113312918293575, 210911076210835, 215517565688425, 294988451482725, 383617980270525, 432759876053505, 442863123838135, 532068058516992, 892813363927485, 923102743748185, 929531173876305 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

10^15 < a(20) <= 1089641067389872.

Also terms: 1248817919303952, 1332436545865422, 1394926716616125, 1868522795664525, 1950445682260072.

a(4) and a(9) appear in Kevin Ford's paper.

LINKS

Table of n, a(n) for n=1..19.

Kevin Ford, Solutions of phi(n)=phi(n+k) and sigma(n)=sigma(n+k), arXiv:2002.12155 [math.NT], 2020.

S. W. Graham, J. J. Holt, and C. Pomerance, On the solutions to phi(n) = phi(n+k), Number Theory in Progress, K. Gyory, H. Iwaniec, and J. Urbanowicz, eds., vol. 2, de Gruyter, Berlin and New York, 1999, pp. 867-882.

Mathematics StackExchange, Conjecture on the gap between integers having the same number of co-primes, Sep 25 2019.

MATHEMATICA

Select[Range[100000], EulerPhi[#] == EulerPhi[# + 3] &] (* Alonso del Arte, Mar 01 2020 *)

PROG

(PARI) isok(k) = eulerphi(k) == eulerphi(k+3); \\ Michel Marcus, Feb 29 2020

CROSSREFS

Cf. A000010, A007015.

Cf. A001274, A001494, A179186, A179187, A179188, A179189, A179202, A330429.

Cf. A276503, A276504, A217139.

Sequence in context: A126334 A068635 A156695 * A175645 A178514 A154924

Adjacent sequences:  A330248 A330249 A330250 * A330252 A330253 A330254

KEYWORD

nonn

AUTHOR

Michel Marcus and Giovanni Resta, Feb 29 2020

STATUS

approved

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Last modified April 11 06:11 EDT 2021. Contains 342886 sequences. (Running on oeis4.)