%N Numbers n such that it is not possible to arrange the numbers from 1 to n in a chain with adjacent links summing to a square.
%C It seems certain, on account of the valences of the underlying graph, that necklaces exist for all larger n, but this may not yet have been proved.
%C The problem originated (for n = 15) with Bernardo Recamán Santos of Colombia. The problem for necklaces is due to Joe Kisenwether.
%D _Ed Pegg Jr_ and _W. Edwin Clark_ have found necklaces (and hence chains) for n = 32 onwards up to 50 and for several larger numbers.
%e E.g., for 15, 16 or 17, use (16-)9-7-2-14-11-5-4-12-13-3-6-10-15-1-8(-17).
%Y Cf. A071983, A071984, A090460, A090461.
%A _R. K. Guy_, Dec 06 2002