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 A059948 #15 Jan 29 2020 19:42:14 %S A059948 0,0,0,0,40,3306,131876,3961356,103290096,2488179582,57162274972, %T A059948 1274774473632,27887396866472,602352276704178,12899161619186388, %U A059948 274612697648135028,5822592730060070368,123107330974129584294 %N A059948 Number of 7-block bicoverings of an n-set. %D A059948 I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983. %H A059948 Vincenzo Librandi, <a href="/A059948/b059948.txt">Table of n, a(n) for n = 1..200</a> %F A059948 a(n)=(1/7!) * (21^n -7*15^n -21*11^n +42*10^n +105*7^n -140*6^n +105*5^n -420*4^n +35*3^n +1050*2^n -1050). %F A059948 The number of m-block bicoverings of an n-set is [x^m*y^n] 1/n!*exp(-x-1/2*x^2*(exp(y)-1)) * sum(i>=0, x^i/i! * exp(binomial(i, 2)*y) ) where [x^m*y^n] extracts the coefficient of x^m*y^n, see Goulden/Jackson p.203. %F A059948 G.f.: 2*x^5*(5197500*x^6-4601880*x^5+1501221*x^4-219455*x^3+12587*x^2+47*x-20) / ((x-1)*(2*x-1)*(3*x-1)*(4*x-1)*(5*x-1)*(6*x-1)*(7*x-1)*(10*x-1)*(11*x-1)*(15*x-1)*(21*x-1)). - _Colin Barker_, Jul 07 2013 %Y A059948 Column k=7 of A059443. %Y A059948 Cf. A002718. %K A059948 easy,nonn %O A059948 1,5 %A A059948 _Vladeta Jovovic_, Feb 14 2001