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 A278933 #21 Jul 06 2025 21:38:47 %S A278933 0,0,0,0,0,0,4,4,8,8,12,12,20,20,24,28,32,32,40,40,48,52,56,56,68,68, %T A278933 72,76,84,84,96,96,104,108,112,116,128,128,132,136,148,148,160,160, %U A278933 168,176,180,180,196,196,204 %N A278933 Number of 2 X 2 matrices with entries in {0,1,...,n} and permanent = trace with no entry repeated. %C A278933 a(n) mod 4 = 0. %H A278933 Chai Wah Wu, <a href="/A278933/b278933.txt">Table of n, a(n) for n = 0..10000</a> (terms for n = 0..285 from Indranil Ghosh) %o A278933 (Python) %o A278933 def t(n): %o A278933 s=0 %o A278933 for a in range(0,n+1): %o A278933 for b in range(0,n+1): %o A278933 for c in range(0,n+1): %o A278933 for d in range(0,n+1): %o A278933 if (a!=b and a!=d and b!=d and c!=a and c!=b and c!=d): %o A278933 if (a*d+b*c)==a+d: %o A278933 s+=1 %o A278933 return s %o A278933 for i in range(0,286): %o A278933 print(t(i), end=', ') %Y A278933 Cf. A210288 (wherein the entries can be repeated). %K A278933 nonn %O A278933 0,7 %A A278933 _Indranil Ghosh_, Dec 02 2016