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 A245789 Rectangular array A read by upward antidiagonals: A(k,n) = (2^k-1)^n, n,k >= 1. 1
 1, 1, 3, 1, 9, 7, 1, 27, 49, 15, 1, 81, 343, 225, 31, 1, 243, 2401, 3375, 961, 63, 1, 729, 16807, 50625, 29791, 3969, 127, 1, 2187, 117649, 759375, 923521, 250047, 16129, 255, 1, 6561, 823543, 11390625, 28629151, 15752961, 2048383, 65025, 511 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS A(k,n) is the number of sequences (X_1, X_2, ..., X_k) of subsets of the set {1, 2, ..., n} such that intersect_{j=1..k} X_j = null. REFERENCES Richard P. Stanley, Enumerative Combinatorics, Cambridge University Press, Vol. 1, Second edition, 2012, p. 14 (Example 1.1.16). LINKS L. Edson Jeffery, Table of n, a(n) for n = 1..45 EXAMPLE Array A begins: 1      3         7           15              31                 63 1      9        49          225             961               3969 1     27       343         3375           29791             250047 1     81      2401        50625          923521           15752961 1    243     16807       759375        28629151          992436543 1    729    117649     11390625       887503681        62523502209 1   2187    823543    170859375     27512614111      3938980639167 1   6561   5764801   2562890625    852891037441    248155780267521 1  19683  40353607  38443359375  26439622160671  15633814156853823 MATHEMATICA (* Array *) a[k_, n_] := (2^k - 1)^n; Grid[Table[a[k, n], {n, 12}, {k, 12}]] (* Array antidiagonals flattened *) Flatten[Table[(2^k - 1)^(n - k + 1), {n, 12}, {k, n}]] CROSSREFS Cf. A000225, A060867, A128831, etc. (rows 1-3). Cf. A000012, A000244, A000420, etc. (columns 1-3). Cf. A055601 (main diagonal). Sequence in context: A197730 A231902 A143495 * A210395 A019770 A136320 Adjacent sequences:  A245786 A245787 A245788 * A245790 A245791 A245792 KEYWORD nonn,tabl AUTHOR L. Edson Jeffery, Aug 22 2014 STATUS approved

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Last modified May 21 14:57 EDT 2019. Contains 323443 sequences. (Running on oeis4.)