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%I #3 Mar 31 2012 13:20:56
%S 1,15,17,1680,421,56086995,9940915081637,
%T 31271643639794562523698024915,
%U 3090319883395626207270178075850946683940891639487741952,9721376888636052447184767296534734059409700304429485542107645657085305
%N a(1) = 1; for n > 1, a(n) is the smallest positive integer not already used such that a(n)*a(n-1) + 1 is a fourth power.
%C The next terms are (to two significant digits) 9.6*10^95, 3.0*10^111, 3.0*10^137, 9.4*10^152, 9.3*10^178.
%e The 4th roots of unity mod 17 are 1, 4, 13 and 16. (4^4 - 1)/17 = 15, which has already been used, so 17 is followed by (13^4 - 1)/17 = 1680.
%Y Cf. A091569, A083203.
%K nonn
%O 1,2
%A _David Wasserman_, May 20 2004