A178953 - OEIS

%I #20 Feb 13 2018 04:04:58

%S 1,2,4,8,9,14,15,21,22,29,30,35,38,46,48,49,50,52,53,57,58,60,61,62,

%T 65,66,90,91,95,96,97,99,114,120,121,122,123,124,125,128,145,146,149,

%U 153,154,163,176,179,180,186,187,189,191,192,197

%N Indices n such that 2*prime(n) cannot be written as a sum of two distinct prime(n-k) and prime(n+k).

%C Snapshots: a(1000) = 6922, a(2000) = 16376, a(3000) = 25951, a(4000) = 37266, a(5000) = 51926, a(6000) = 69928. - R. J. Mathar, Jan 08 2011

%H Reinhard Zumkeller, <a href="/A178953/b178953.txt">Table of n, a(n) for n = 1..500</a>

%F A178609(a(n))=0.

%p A178609 := proc(n) for k from n-1 to 0 by -1 do if ithprime(n-k)+ithprime(n+k)=2*ithprime(n) then return k; end if; end do: end proc:

%p for n from 1 to 200 do if A178609(n) = 0 then printf("%d,",n) ; end if; end do: # _R. J. Mathar_, Jan 05 2011

%t A178609[n_] := For[k = n-1, k >= 0, k--, If[Prime[n-k] + Prime[n+k] == 2*Prime[n], Return[k]]]; Reap[For[n = 1, n <= 200, n++, If[A178609[n] == 0, Sow[n]]]][[2, 1]] (* _Jean-François Alcover_, Feb 13 2018, after _R. J. Mathar_ *)

%o (Haskell)

%o a178953 n = a178953_list !! (n-1)

%o a178953_list = filter ((== 0) . a178609) [1..]

%o -- _Reinhard Zumkeller_, Jan 30 2014

%Y Cf. A178609, A071681, A178954.

%K nonn

%O 1,2

%A _Juri-Stepan Gerasimov_, Jan 02 2011