%I #4 Mar 31 2012 12:35:46
%S 0,6,42,166,612,1649,4501,9772,21631,39911,78602,131634,234593,361145,
%T 603810,883492,1396448,1928226,2949092,3948312,5771436,7496953,
%U 10742472,13520820,18931459,23338564,31813882,38799595,52158316,61609006,82337294
%N Number of distinct solutions of sum{i=1..4}(x(2i-1)*x(2i)) = 1 (mod n), with x() in 0..n-1
%C Column 4 of A180813
%H R. H. Hardin, <a href="/A180806/b180806.txt">Table of n, a(n) for n=1..376</a>
%e Solutions for sum of products of 4 0..1 pairs = 1 (mod 2) are
%e (0*0 + 0*0 + 0*0 + 1*1) (0*0 + 0*0 + 0*1 + 1*1) (0*0 + 0*1 + 0*1 + 1*1)
%e (0*0 + 1*1 + 1*1 + 1*1) (0*1 + 0*1 + 0*1 + 1*1) (0*1 + 1*1 + 1*1 + 1*1)
%K nonn
%O 1,2
%A _R. H. Hardin_, suggested by _Max Alekseyev_ in the Sequence Fans Mailing List, Sep 20 2010