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 A118877 #13 Aug 03 2020 19:49:17
%S A118877 -12,-12,6,6,-18,-18,12,12,-24,-24,18,-3,-28,-2,-2,24,24,-36,-36,-2,
%T A118877 -2,32,-3,-42,36,36,-48,-48,42,-3,-52,-2,-2,48,-3,-58,-2,-2,54,54,-66,
%U A118877 -66,-2,-2,62,-3,-72,66,66,-78,-78,-2,-2,74,-3,-84,78,-3,-88,-2,-2
%N A118877 Determinant of n-th continuous block of 4 consecutive composites.
%C A118877 Composites analog of A117301 Determinants of 2 X 2 matrices of continuous blocks of 4 consecutive primes. See also: A118780 Determinants of 2 X 2 matrices of continuous blocks of 4 consecutive semiprimes. The terminology "continuous" is used to distinguish from "discrete" which would be (in this composites case) block 1: 4, 6, 8, 9; block 2: 10, 12, 14, 15 and so forth. It is not until a(12) that we break the pattern of a(2n)=a(2n-1); there seem to be strangely many such pairs of two identical values. a(12) is also the first odd value in the sequence and the first prime.
%H A118877 Harvey P. Dale, <a href="/A118877/b118877.txt">Table of n, a(n) for n = 1..1000</a>
%F A118877 a(n) = A002808(n)*A002808(n+3) - A002808(n+1)*A002808(n+2).
%e A118877 a(1) = -12 =
%e A118877 |4 6|
%e A118877 |8 9|.
%t A118877 Module[{nn=100,cmps},cmps=Select[Range[nn],CompositeQ];Det[ ArrayReshape[ #,{2,2}]]&/@Table[Take[cmps,{n,n+3}],{n,Length[cmps]-3}]] (* _Harvey P. Dale_, Aug 03 2020 *)
%Y A118877 Cf. A002808, A117301, A118780.
%K A118877 easy,sign
%O A118877 1,1
%A A118877 _Jonathan Vos Post_, May 24 2006