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 A107983 Triangle read by rows: T(n,k) = (k+1)(n+2)(n+3)(n-k+2)(n-k+1)/12 for 0<=k<=n. 0
 1, 6, 4, 20, 20, 10, 50, 60, 45, 20, 105, 140, 126, 84, 35, 196, 280, 280, 224, 140, 56, 336, 504, 540, 480, 360, 216, 84, 540, 840, 945, 900, 750, 540, 315, 120, 825, 1320, 1540, 1540, 1375, 1100, 770, 440, 165, 1210, 1980, 2376, 2464, 2310, 1980, 1540 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS KekulĂ© numbers for certain benzenoids. Column 0 yields A002415. Main diagonal yields A000292. Row sums yield A006542. T(n,k) = number of Dyck (n+4)-paths with 4 peaks (UDs) and last descent of length k+1. For example, T(1,1)=4 counts UUDUDUDUDD, UDUUDUDUDD, UDUDUUDUDD, UDUDUDUUDD. - David Callan, Jun 26 2006 REFERENCES S. J. Cyvin and I. Gutman, KekulĂ© structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (p. 237, K{F(n,3,-l)}). LINKS EXAMPLE Triangle begins: 1; 6,4; 20,20,10; 50,60,45,20; MAPLE T:=proc(n, k) if k<=n then (k+1)*(n+2)*(n+3)*(n-k+2)*(n-k+1)/12 else 0 fi end: for n from 0 to 10 do seq(T(n, k), k=0..n) od; # yields sequence in triangular form MATHEMATICA Flatten[Table[((k+1)(n+2)(n+3)(n-k+2)(n-k+1))/12, {n, 0, 10}, {k, 0, n}]] (* Harvey P. Dale, Aug 08 2013 *) CROSSREFS Cf. A002415, A000292, A006542. Sequence in context: A160248 A317858 A212891 * A009278 A213573 A321417 Adjacent sequences:  A107980 A107981 A107982 * A107984 A107985 A107986 KEYWORD nonn,tabl AUTHOR Emeric Deutsch, Jun 12 2005 STATUS approved

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Last modified May 13 19:36 EDT 2021. Contains 343868 sequences. (Running on oeis4.)