A221704 - OEIS

%I #7 Jan 24 2013 03:34:57

%S 20,96,440,1956,8522,36616,155748,657468,2759040,11523584,47944480,

%T 198834824,822365998,3393330152,13973673788,57441828332,235759507928,

%U 966292212992,3955570350128,16174269135992,66069496614548,269635886185424,1099484607270920,4479870682221016,18240349141736352,74219083904054016,301810005706454016

%N Let K be a local ring with a principal maximal ideal J of nilpotent degree 4 with |K/J|>2; a(n) = number of D-invariant ideals in the ring R_n(K,J).

%D G. P. Egorychev et al., Enumeration of ideals of some nilpotent matrix rings, J. Algebra and Applications, 12 (2013), #1250140.

%p f:=(n,s)->(2*s*n-s-3*n+1)*binomial(2*n-2,n-1)-(4/n)*binomial(2*n,n-2)+2^(2*n-1);

%p [seq(f(n,4),n=2..40)];

%o (Maxima) A221704(n, s):=(2*s*n-s-3*n+1)*binomial(2*n-2, n-1)-(4/n)*binomial(2*n, n-2)+2^(2*n-1)$

%o makelist(A221704(n,4),n,2,40); /* _Martin Ettl_, Jan 24 2013 */

%K nonn

%O 2,1

%A _N. J. A. Sloane_, Jan 22 2013