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

1,1

Links

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

Formula

p in A028416, but not A001913.

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 *)

Prog

(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

Crossrefs

Cf. A028416.

Sequence in context: A132201 A057189 A072580 * A226242 A116436 A185240

Adjacent sequences: A186637 A186638 A186639 * A186641 A186642 A186643

Keyword

nonn,base

Author

Jani Melik, Feb 24 2011

Status

approved