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A276052 Least k > 1 such that phi(k*n-1) = phi(k*n+1), or -1 if no such k exists. 1
5, 4, 3, 2, 15, 106, 21, 127, 3, 39282, 3, 53, 135, 65014, 5, 9489, 171, 361, 27, 19641, 7, 13133, 141, 6326, 3, 6978, 19, 32507, 375, 13094, 165, 93186, 19, 1359, 9, 12588, 15, 171, 45, 35253, 3, 35794, 9, 16796, 7, 1689, 69, 3163, 3, 13653, 57, 3489, 12, 249, 45, 58497, 9 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Least k > 1 such that k*n is in A066812. - Robert Israel, Aug 30 2016

LINKS

Table of n, a(n) for n=1..57.

EXAMPLE

a(5) = 15 because phi(15*5-1) = phi(15*5+1).

MAPLE

f:= proc(n) local k;

   for k from 2 do if numtheory:-phi(k*n-1) = numtheory:-phi(k*n+1) then

      return k

   fi od end proc:

map(f, [$1..60]); # Robert Israel, Aug 30 2016

MATHEMATICA

kmax = 10^9;

a[n_] := Module[{k}, For[k = 2, k <= kmax, k++, If[EulerPhi[k n - 1] == EulerPhi[k n + 1] , Print[n, " ", k]; Return[k]]]; -1];

Array[a, 60] (* Jean-Fran├žois Alcover, Oct 06 2020 *)

PROG

(PARI) a(n) = {my(k = 2); while (eulerphi(k*n+1) != eulerphi(k*n-1), k++); k; }

CROSSREFS

Cf. A000010, A066812, A275412.

Sequence in context: A145330 A194744 A132669 * A321028 A263356 A061836

Adjacent sequences:  A276049 A276050 A276051 * A276053 A276054 A276055

KEYWORD

nonn

AUTHOR

Altug Alkan, Aug 17 2016

EXTENSIONS

Name corrected by Robert Israel, Aug 30 2016

STATUS

approved

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Last modified June 22 15:24 EDT 2021. Contains 345386 sequences. (Running on oeis4.)