A154119 Numbers k such that binomial(3k,k) is odd and not divisible by 5.

0, 1, 5, 8, 16, 33, 40, 41, 65, 66, 80, 81, 128, 130, 133, 138, 165, 325, 328, 330, 640, 641, 650, 656, 658, 1025, 1026, 1028, 1033, 1040, 1041, 2065, 2066, 2080, 2081, 4128, 4130, 4133, 4138, 4165, 4688, 4690, 4753, 4756, 4776, 4778, 5125, 5128, 5130, 5138

Offset: 0

Author

Zak Seidov, Jan 05 2009

Keywords

• nonn

Comments

From the first 10001 terms of A005809 only 80 are odd and not divisible by 5.

Links

• Chai Wah Wu, Table of n, a(n) for n = 0..10000

Crossrefs

Cf. A125221 Numbers n such that binomial(3n,n)+1 is prime, A005809 Binomial coefficients C(3n,n).

Programs

Mathematica

(*M6*)Reap[Do[b=Binomial[3n,n];If[OddQ[b]&&Mod[b,10]=/=5,Sow[n]],{n,100}]][[2,1]]
bQ[n_]:=Module[{bi=Binomial[3n,n]},OddQ[bi]&&!Divisible[bi,5]]; Select[Range[0,5300],bQ] (* Harvey P. Dale, Jul 19 2011 *)

Formula

A005809(a(n)) is odd and not divisible by 5.

Extensions

Corrected definition - Harvey P. Dale, Jul 19 2011