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URL: https://oeis.org/A179187

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A179187
Numbers n such that phi(n)=phi(n+5), with Euler's totient function phi=A000010.
10
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
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.
Kevin Ford, Solutions of phi(n)=phi(n+k) and sigma(n)=sigma(n+k), arXiv:2002.12155 [math.NT], 2020.
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
KEYWORD
nonn
AUTHOR
M. F. Hasler, Jan 05 2011
STATUS
approved