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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.

A278933 Number of 2 X 2 matrices with entries in {0,1,...,n} and permanent = trace with no entry repeated.

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%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