A186640 Primes p such that the decimal expansion of 1/p has a periodic part of even length, but are not cyclic numbers (A001913).

11, 13, 73, 89, 101, 103, 127, 137, 139, 157, 197, 211, 241, 251, 281, 293, 331, 349, 353, 373, 401, 409, 421, 449, 457, 463, 521, 557, 569, 601, 607, 617, 641, 653, 661, 673, 677, 691, 739, 761, 769, 809, 829, 859, 877, 881, 929, 967, 997, 1009, 1049, 1061

Offset: 1

Author

Jani Melik, Feb 24 2011

Keywords

• nonn
• base

Links

• T. D. Noe, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A028416.

Programs

Maple

f1_d := proc(n) local st, period:
st := ithprime(n):
period := numtheory[order](10,st):
if (modp(period,2) = 0) then
   if (st-1 <> period) then
      RETURN(st):
   fi:
fi: end:  seq(f1_d(n), n=1..200);

Mathematica

Select[Prime[Range[200]], EvenQ[Length[RealDigits[1/#][[1, 1]]]] && MultiplicativeOrder[10, #] != # - 1 &] (* T. D. Noe, Oct 01 2012 *)

PARI

is(p)=if(p>9 && isprime(p), my(o=znorder(Mod(10, p))); o%2==0 && o+1!=p, 0) \\ Charles R Greathouse IV, Oct 01 2012

Formula

p in A028416, but not A001913.