1,2

Comments from Olivier GERARD (olivier.gerard(AT)gmail.com), Sep 18 2007: (Start) Number of classes of permutations arrays giving the same partition by the following transformation (equivalent in this case to X-rays but more general): on the matrix representation of a permutation of order n, the sum (i.e. here number of ones) in the i-th antidiagonal is the number of copies of i in the partition.

This gives an injection of permutations of n into partitions with parts at most 2n-1. The first or the last antidiagonal can be omitted, reducing the size of parts to 2n-2 without changing the number of classes.

This sequence is called Lambda_{n,1} in Louck's paper and appears explicitely in p758. Terms up to 10 were computed by Myron Stein (arXiv).

This is similar to the number of Schur functions studied by Di Francesco and al. (A007747) related to the powers of the Vandermonde determinant. Also number of classes of straight (monotonic) crossing bi-permutations. (End)

Olivier Gerard and Karol Penson, Set partitions, multiset permutations and bi-permutations, in preparation.

James D. Louck, Power of a determinant with two physical applications, Internat. J. Math. & Math. Sci., Vol. 22, No 4(1999) p745-759 - S 0161-1712(99)22745-7

C. Bebeacua, T. Mansour, A. Postnikov and S. Severini, On the x-rays of permutations

J.-P. Davalan, Permutations et tomographie - X-rays.

Adjacent sequences: A019586 A019587 A019588 this_sequence A019590 A019591 A019592

Sequence in context: A019448 A000753 A007878 this_sequence A087949 A028333 A007747

nonn,nice

Alex Postnikov (apost(AT)math.mit.edu)

More terms from Olivier GERARD (olivier.gerard(AT)gmail.com), Sep 18 2007

Two more terms from Vladeta Jovovic (vladeta(AT)eunet.rs), Oct 04 2007