# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a281423 Showing 1-1 of 1 %I A281423 #6 Jan 22 2017 21:53:15 %S A281423 0,0,0,1,2,1,0,2,2,0,2,3,0,0,2,0,0,3,2,0,0,2,2,2,2,0,2,0,0,2,0,3,2,0, %T A281423 0,4,4,0,2,2,0,1,2,2,2,2,0,2,0,2,4,0,0,0,2,0,2,4,0,1,2,0,2,4,0,2,2,1, %U A281423 0,2,2,2,2,0,0,4,0,0,0,2,2,2,0,1,6,0,0,2,2,0,0,2,2,2,2,2,2,4,4,2,0,2,0,0,2,4,0,2,4,1,2,4 %N A281423 Expansion of (Sum_{k>=1} x^prime(prime(k)))^2 [even terms only]. %C A281423 Number of ways to write 2n as an ordered sum of two primes with prime subscripts (A006450). %H A281423 Ilya Gutkovskiy, Extended graphical example %H A281423 Index entries for sequences related to compositions %F A281423 G.f.: (Sum_{k>=1} x^prime(prime(k)))^2 [even terms only]. %e A281423 a(4) = 2 because we have [3, 5] and [5, 3], where 3 = prime(2) = prime(prime(1)) and 5 = prime(3) = prime(prime(2)). %t A281423 Take[CoefficientList[Series[Sum[x^Prime[Prime[k]], {k, 1, 250}]^2, {x, 0, 250}], x], {1, -1, 2}] %Y A281423 Cf. A001031, A002375, A006450, A045917, A073610, A281422. %K A281423 nonn %O A281423 0,5 %A A281423 _Ilya Gutkovskiy_, Jan 21 2017 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE