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A179186 Numbers n such that phi(n) = phi(n+4), with Euler's totient function phi=A000010. 10
8, 14, 16, 20, 35, 52, 64, 91, 140, 148, 244, 292, 403, 455, 616, 628, 772, 801, 1011, 1024, 1108, 1144, 1252, 1270, 1295, 1456, 1588, 1684, 1820, 1828, 2030, 2164, 2452, 2623, 2644, 2660, 2692, 2932, 3028, 3216, 3321, 3508, 3988, 4264, 4340, 4372, 4612, 4804, 4852, 4948 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Is there some n > 5 such that phi(n) = phi(n+3)?

None up to 500000. (* Harvey P. Dale, Feb 16 2011 *)

No further solutions to the phi(n) = phi(n+3) problem less than 10^12.  On the other hand, this sequence has 267797240 terms under 10^12. - Jud McCranie, Feb 13 2012

No reason is known that would prevent other solutions of phi(n)=phi(n+3), see Graham, Holt, & Pomerance. - Jud McCranie, Jan 03 2013

If a(n) is even then a(n)/2 is in A001494 - see comment at A217139. - Jud McCranie, Dec 31 2012

REFERENCES

S. W. Graham, J. J. Holt, & C. Pomerance, "On the solutions to phi(n)=phi(n+k)", Number Theory in Progress, Proc. Intern. Conf. in Honor of 60th Birthday of A. Schinzel, Poland, 1997.  Walter de Gruyter, 1999, pp. 867-82.

LINKS

T. D. Noe and Jud McCranie, Table of n, a(n) for n = 1..10000 (first 1000 terms from T. D. Noe)

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

MATHEMATICA

Select[Range[5000], EulerPhi[#]==EulerPhi[#+4]&]  (* Harvey P. Dale, Feb 16 2011 *)

PROG

(PARI) {op=vector(N=4); for( n=1, 1e4, if( op[n%N+1]+0==op[n%N+1]=eulerphi(n), print1(n-N, ", ")))}

(MAGMA) [n: n in [1..5000] | EulerPhi(n) eq EulerPhi(n+4)]; // Vincenzo Librandi, Sep 08 2016

CROSSREFS

Cf. A000010, A001274, A001494, A179187, A007015.

Sequence in context: A050681 A292867 A235143 * A192045 A084021 A138666

Adjacent sequences:  A179183 A179184 A179185 * A179187 A179188 A179189

KEYWORD

nonn

AUTHOR

M. F. Hasler, Jan 05 2011

STATUS

approved

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Last modified May 24 20:53 EDT 2019. Contains 323534 sequences. (Running on oeis4.)