This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A068718 #9 Jun 15 2022 01:50:18 %S A068718 1,2,2,2,8,25,90,404,2055,11792,75053,525622,4015361,33231679, %T A068718 296182315,2828335731,28809181418,311788442591,3572832236720, %U A068718 43216177809190,550245463265240,7356239983352887,103028812072639378 %N A068718 Boustrophedon transform of A068717 with A068717(1) = -1 instead of 0. %C A068718 Signs omitted, all terms < 0. A068717(1) = -1 means soluble (1*1 -1*0*0 = +1 resp. 0*0 -1*1*1 = -1). %o A068718 (Python) %o A068718 from itertools import accumulate, count, islice %o A068718 from math import isqrt %o A068718 from sympy import continued_fraction_periodic %o A068718 def A068718_gen(): # generator of terms %o A068718 yield 1 %o A068718 blist = (1,) %o A068718 for n in count(2): %o A068718 yield (blist := tuple(accumulate(reversed(blist),initial=0 if (a:=isqrt(n)**2==n) else (1 if len(continued_fraction_periodic(0,1,n)[1]) & 1 else int(a)-1))))[-1] %o A068718 A068718_list = list(islice(A068718_gen(),40)) # _Chai Wah Wu_, Jun 14 2022 %Y A068718 Cf. A068716, A068717, A049240, A046951. %K A068718 nonn,easy %O A068718 0,2 %A A068718 _Frank Ellermann_, Feb 26 2002