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A179187 Numbers n such that phi(n)=phi(n+5), with Euler's totient function phi=A000010. 8
5, 9, 15, 21, 15556, 21016, 25930, 25935, 27027, 36304, 46683, 129675, 266128, 307923, 329175, 430348, 503139, 636400, 684411, 812170, 1014778, 1252713, 1777545, 1871788, 1892452, 1911987, 2622160, 2629930, 2731360, 2947035, 3397480, 4200100, 5451537 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

There are only 43 terms below 10^7, and 1843 terms below 10^12. [Jud McCranie, Feb 13 2012]

LINKS

Jud McCranie, Table of n, a(n) for n = 1..1843 (terms < 10^12)

F. Firoozbakht, Puzzle 466. phi(n-1)=phi(n)=phi(n+1), in C. Rivera's Primepuzzles.

FORMULA

A000010(a(n)) = A000010(a(n)+5).

MATHEMATICA

Select[Range[5000000], EulerPhi[#] == EulerPhi[# + 5] &] (* Vincenzo Librandi, Sep 08 2016 *)

Position[Partition[EulerPhi[Range[55*10^5]], 6, 1], _?(#[[1]] == #[[6]]&), {1}, Heads->False]//Flatten (* Harvey P. Dale, Nov 10 2016 *)

PROG

(PARI) {op=vector(N=5); for( n=1, 1e7, if( op[n%N+1]+0==op[n%N+1]=eulerphi(n), print1(n-N, ", ")))}

(MAGMA) [n: n in [1..5000000] | EulerPhi(n) eq EulerPhi(n+5)]; // Vincenzo Librandi, Sep 08 2016

CROSSREFS

Cf. A000010, A001274, A001494, A179186, A007015.

Sequence in context: A315079 A217277 A087679 * A315080 A315081 A075343

Adjacent sequences:  A179184 A179185 A179186 * A179188 A179189 A179190

KEYWORD

nonn

AUTHOR

M. F. Hasler, Jan 05 2011

STATUS

approved

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Last modified May 19 08:59 EDT 2019. Contains 323389 sequences. (Running on oeis4.)