A129631 - OEIS

%I #8 Aug 13 2014 09:47:10

%S 2,6,15,20,24,31,34,43,52,61,67,70,76,85,88,91,94,97,103,106,112,115,

%T 121,124,127,130,133,136,139,141,145,151,154,160,163,166,169,178,181,

%U 190,193,196,199,200,208,211,217,226,229,232,235,238,241,247,250,259

%N Numbers n such that sum of digits of binomial(n+1,n-1) is a prime.

%H Harvey P. Dale, <a href="/A129631/b129631.txt">Table of n, a(n) for n = 1..1000</a>

%e binomial(6+1,6-1)=binomial(7,5)=7!/(5!*2!)=21 --> 2+1=3 prime.

%p P:=proc(n) local i,k,w; for i from 1 by 1 to n do w:=0; k:=binomial(i+1,i-1); while k>0 do w:=w+k-(trunc(k/10)*10); k:=trunc(k/10); od; if isprime(w) then print(i); fi; od; end: P(1000);

%t Select[Range[300],PrimeQ[Total[IntegerDigits[Binomial[#+1,#-1]]]]&] (* _Harvey P. Dale_, Aug 13 2014 *)

%K nonn,base

%O 1,1

%A _Paolo P. Lava_ and _Giorgio Balzarotti_, May 31 2007