A050797 - OEIS

%I #18 Jun 20 2015 11:38:10

%S 3,9,17,19,33,35,73,145,161,163,195,243,393,483,513,721,723,1153,1763,

%T 2177,2305,2593,4803,5185,5833,6273,6963,7057,7395,8713,9523,9603,

%U 10083,12483,13923,14113,15875,17425,17673,19043,20737

%N Numbers n such that n^2 - 1 is expressible as the sum of two nonzero squares in exactly one way.

%C If the definition were changed from "nonzero squares" to "nonnegative squares", there would be just one additional term, 1. - _T. D. Noe_, May 27 2008

%H T. D. Noe, <a href="/A050797/b050797.txt">Table of n, a(n) for n=1..300</a>

%H Eric Weisstein, <a href="http://mathworld.wolfram.com/SumofSquaresFunction.html">MathWorld: Sum of Squares Function</a>

%H <a href="/index/Su#ssq">Index entries for sequences related to sums of squares</a>

%e E.g. 393^2 - 1 = 28^2 + 392^2 only.

%t twoSquaresQ[ n_] := (r = Reduce [0 < a <= b && n^2 - 1 == a^2 + b^2, {a, b}, Integers]; Head[r] === And); Select[ Range[21000], twoSquaresQ] (* _Jean-François Alcover_, Oct 10 2011 *)

%Y Cf. A050798, A050795.

%Y Cf. A000161

%K nonn,nice

%O 1,1

%A _Patrick De Geest_, Sep 15 1999

%E More terms from _James A. Sellers_