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Numbers n such that binomial(3n,n) is odd and not divisible by 5.
1

%I #11 Feb 10 2016 23:12:09

%S 0,1,5,8,16,33,40,41,65,66,80,81,128,130,133,138,165,325,328,330,640,

%T 641,650,656,658,1025,1026,1028,1033,1040,1041,2065,2066,2080,2081,

%U 4128,4130,4133,4138,4165,4688,4690,4753,4756,4776,4778,5125,5128,5130,5138

%N Numbers n such that binomial(3n,n) is odd and not divisible by 5.

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

%H Chai Wah Wu, <a href="/A154119/b154119.txt">Table of n, a(n) for n = 0..4000</a>

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

%t (*M6*)Reap[Do[b=Binomial[3n,n];If[OddQ[b]&&Mod[b,10]=/=5,Sow[n]],{n,100}]][[2,1]]

%t bQ[n_]:=Module[{bi=Binomial[3n,n]},OddQ[bi]&&!Divisible[bi,5]]; Select[Range[0,5300],bQ] (* _Harvey P. Dale_, Jul 19 2011 *)

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

%K nonn

%O 0,3

%A _Zak Seidov_, Jan 05 2009

%E Corrected definition - _Harvey P. Dale_, Jul 19 2011