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 A224332 #16 Aug 31 2018 15:59:43
%S A224332 1,30,381,4092,40955,393210,3670009,33554424,301989879,2684354550,
%T A224332 23622320117,206158430196,1786706395123,15393162788850,
%U A224332 131941395333105,1125899906842608,9570149208162287,81064793292668910,684547143360315373
%N A224332 Number of idempotent n X n 0..7 matrices of rank n-1.
%C A224332 Column 7 of A224333.
%H A224332 R. H. Hardin, <a href="/A224332/b224332.txt">Table of n, a(n) for n = 1..210</a>
%H A224332 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (18,-97,144,-64).
%F A224332 a(n) = n*(2*8^(n-1)-1).
%F A224332 a(n) = 18*a(n-1) - 97*a(n-2) + 144*a(n-3) - 64*a(n-4).
%F A224332 G.f.: x*(1 + 12*x - 62*x^2) / ((1 - x)^2*(1 - 8*x)^2). - _Colin Barker_, Aug 29 2018
%e A224332 Some solutions for n=3:
%e A224332 ..1..0..3....0..0..4....0..3..0....0..0..1....1..0..6....0..7..7....1..0..3
%e A224332 ..0..1..3....0..1..0....0..1..0....0..1..0....0..1..4....0..1..0....0..1..6
%e A224332 ..0..0..0....0..0..1....0..0..1....0..0..1....0..0..0....0..0..1....0..0..0
%t A224332 Table[n*(2*8^(n-1)-1), {n, 1, 40}] (* _Stefano Spezia_, Aug 29 2018 *)
%o A224332 (PARI) Vec(x*(1 + 12*x - 62*x^2) / ((1 - x)^2*(1 - 8*x)^2) + O(x^40)) \\ _Colin Barker_, Aug 29 2018
%o A224332 (PARI) a(n) = n*(2*8^(n-1)-1); \\ _Altug Alkan_, Aug 31 2018
%Y A224332 Cf. A224333.
%K A224332 nonn,easy
%O A224332 1,2
%A A224332 _R. H. Hardin_, formula via _M. F. Hasler_ _William J. Keith_ and Rob Pratt in the Sequence Fans Mailing List, Apr 03 2013