OFFSET
1,1
COMMENTS
According to the conjecture in A242753, this sequence should have infinitely many terms.
Conjecture: The number of terms not exceeding x > 1 has the main term x/(log x) as x tends to infinity.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..10000
EXAMPLE
a(4) = 6 since 6*11 == 1 (mod prime(6)=13) with 11 prime, but 5*9 == 1 (mod prime(5)=11) with 9 composite.
MATHEMATICA
p[n_]:=PrimeQ[PowerMod[n, -1, Prime[n]]]
n=0; Do[If[p[k], n=n+1; Print[n, " ", k]]; Continue, {k, 1, 247}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, May 22 2014
STATUS
approved