OFFSET
1,1
COMMENTS
Least k > 1 such that k*n is in A066812. - Robert Israel, Aug 30 2016
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-François Alcover, Oct 06 2020 *)
PROG
(PARI) a(n) = {my(k = 2); while (eulerphi(k*n+1) != eulerphi(k*n-1), k++); k; }
CROSSREFS
KEYWORD
nonn
AUTHOR
Altug Alkan, Aug 17 2016
EXTENSIONS
Name corrected by Robert Israel, Aug 30 2016
STATUS
approved