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 A194460 a(n) is the number of basic ideals in the standard Borel subalgebra of the untwisted affine Lie algebra sl_n. 4
 1, 4, 18, 82, 370, 1648, 7252, 31582, 136338, 584248, 2488156, 10540484, 44450068, 186715072, 781628008, 3262239862, 13579324498, 56391614632, 233686316428, 966556003132, 3990942300508, 16453094542432, 67733512006168 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n) also equals the number of pairs (p,q) of Dyck paths of semilength n, such that the first peak of q has height at least n-l(p), where l(p) is the height of the last peak of p, and the last peak of q has height at least n-f(p), where f(p) is the height of the first peak of p. From Per Alexandersson, May 26 2018: (Start) a(n) is also equal to the number of circular arc digraphs on n vertices. a(n) is equal to the number of lists b(1),b(2),...,b(n) such that 0 <= b(i) < n and b(i)-1 <= b(i+1) for i=1..n-1 and b(n)-1 <= b(1). The subset of such sequences such that b(n)=0 is given by the Catalan numbers, A000108. (End) Christian Krattenthaler has shown that a(n) = (n+2)*binomial(2*n-1,n-1) - 2^(2*n-1), which also implies the above recursion observed by D. S. McNeil. - Volodymyr Mazorchuk, Aug 26 2011 LINKS G. C. Greubel, Table of n, a(n) for n = 1..1000 Per Alexandersson, Svante Linusson, Samu Potka, The cyclic sieving phenomenon on circular Dyck paths, arXiv:1903.01327 [math.CO], 2019. Per Alexandersson and Greta Panova, LLT polynomials, chromatic quasisymmetric functions and graphs with cycles, arXiv:1705.10353 [math.CO], 2017. See Lemma 5. K. Baur and V. Mazorchuk; Combinatorial analogues of ad-nilpotent ideals for untwisted affine Lie algebras, arXiv:1108.3659 [math.RA], 2011. FORMULA It appears that the sequence is given by a(1)=1, a(n) = 4*a(n-1) + 2*binomial(2*n-3, n-3). - D. S. McNeil, Aug 25 2011 0 =  a(n)*(+2304*a(n+1) -3744*a(n+2) +1464*a(n+3) -168*a(n+4)) +a(n+1)*(-96*a(n+1) +1192*a(n+2) -730*a(n+3) +102*a(n+4)) +a(n+2)*(-78*a(n+2) +99*a(n+3) -19*a(n+4)) +a(n+3)*(-3*a(n+3) +a(n+4)) for all n>0. - Michael Somos, Jun 28 2018 EXAMPLE G.f. = x + 4*x^2 + 18*x^3 + 82*x^4 + 370*x^5 + 1648*x^6 + 7252*x^7 + 31582*x^8 + ... - Michael Somos, Jun 28 2018 MATHEMATICA a[n_] := (n+2) Binomial[2n-1, n-1] - 2^(2n-1); Array[a, 23] (* Jean-François Alcover, Jul 27 2018, after Michael Somos *) PROG (Sage) def A194460(n):     if n == 1: return 1     cf = CachedFunction(lambda i, j, n: binomial(n-1-i+n-1-j, n-i-1)-binomial(n-1-i+n-1-j, n-i-j-1))     CP = cartesian_product     return sum(sum(cf(i, j, n)*cf(k, m, n) for k, m in CP([[n-i..n], [n-j..n]])) for i, j in CP([[1..n], [1..n]])) # D. S. McNeil, Aug 25 2011 (PARI) {a(n) = if( n<1, 0, (n+2) * binomial(2*n-1, n-1) - 2^(2*n-1))}; /* Michael Somos, Jun 28 2018 */ (MAGMA) [(n+2)*Binomial(2*n-1, n-1) - 2^(2*n-1): n in [1..30]]; // G. C. Greubel, Aug 13 2018 CROSSREFS Sequence in context: A181610 A264927 A257059 * A100192 A052913 A279285 Adjacent sequences:  A194457 A194458 A194459 * A194461 A194462 A194463 KEYWORD nonn AUTHOR Volodymyr Mazorchuk, Aug 24 2011 EXTENSIONS More terms from D. S. McNeil, Aug 25 2011 STATUS approved

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Last modified November 27 18:22 EST 2020. Contains 338683 sequences. (Running on oeis4.)