%I #2 Mar 30 2012 17:26:23
%S 1,3,2,7,4,5,6,10,8,17,9,16,11,14,12,13,15,21,18,31,19,30,20,29,22,27,
%T 23,26,24,25,28,36,32,49,33,48,34,47,35,46,37,44,38,43,39,42,40,41,45,
%U 55,50,71,51,70,52,69,53,68,54,67,56,65,57,64,58,63,59,62,60,61,66,78
%N Rearrangement of natural numbers such that each two terms sum up to a perfect square.
%e 1+3=4, 2+7=9, 4+5=9, 6+10=16, 8+17=25.
%t s={};ra=Range[2000]; Do[su=ra[[1]]; c=2;While[ !IntegerQ[Sqrt[su+ra[[c]]]],c++ ];rac=ra[[c]]; s=Join[s,{ra[[1]],rac}];ra=Complement[ra,{ra[[1]],rac}],{133}];s
%Y Cf. A130108, A130110, A130111.
%K nonn
%O 1,2
%A _Zak Seidov_, May 08 2007
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