Card deals, lattice paths, abelian words and combinatorial identities

We give combinatorial interpretations of several related identities associated with the names Barrucand, Strehl and Franel, including one for the Apery numbers. The combinatorial constructs employed are derangement-type card deals as introduced in a previous paper on Barrucand's identity, labeled lattice paths and, following a comment of Jeffrey Shallit, abelian words over a 3-letter alphabet.Comment: 10 pages, LaTe

Jordan and Smith forms of Pascal-related matrices

We present matrix identities which yield respectively the Jordan canonical form of the Pascal matrix P_n = (i -1 choose j -1)_{1 <= i,j <= n} modulo a prime, the eigenvectors of (i choose j)_{1 <= i,j <= n}, and the Smith normal form of powers of P_n - I_n.Comment: LaTeX, 4 page

On conjugates for set partitions and integer compositions

There is a familiar conjugate for integer partitions: transpose the Ferrers diagram, and a conjugate for integer compositions: transpose a Ferrers-like diagram. Here we propose a conjugate for set partitions and show that it interchanges # singletons and # adjacencies. Its restriction to noncrossing partitions cropped up in a 1972 paper of Kreweras. We also exhibit an analogous pairs of statistics interchanged by the composition conjugate.Comment: Improved definition of conjugate partition; noncrossing partitions considered. 7 page

A sign-reversing involution to count labeled lone-child-avoiding trees

We use a sign-reversing involution to show that trees on the vertex set [n], considered to be rooted at 1, in which no vertex has exactly one child are counted by 1/n sum_{k=1}^{n} (-1)^(n-k) {n}-choose-{k} (n-1)!/(k-1)! k^(k-1). This result corrects a persistent misprint in the Encyclopedia of Integer Sequences.Comment: 4 page

Counting stabilized-interval-free permutations

A stabilized-interval-free (SIF) permutation on [n]={1,2,...,n} is one that does not stabilize any proper subinterval of [n]. By presenting a decomposition of an arbitrary permutation into a list of SIF permutations, we show that the generating function A(x) for SIF permutations satisfies the defining property: [x^(n-1)] A(x)^n = n! . We also give an efficient recurrence for counting SIF permutations.Comment: latex, 6 page

The number of bar{3}bar{1}542-avoiding permutations

We confirm a conjecture of Lara Pudwell and show that permutations of [n] that avoid the barred pattern bar{3}bar{1}542 are counted by OEIS sequence A047970. In fact, we show bijectively that the number of bar{3}bar{1}542 avoiders of length n with j+k left-to-right maxima, of which j initiate a descent in the permutation and k do not, is {n}-choose-{k} j! StirlingPartition{n-j-k}{j}, where StirlingPartition{n}{j} is the Stirling partition number.Comment: 5 pages, 2 figures, minor correction

Kreweras's Narayana number identity has a simple Dyck path interpretation

We show that an identity of Kreweras for the Narayana numbers counts Dyck paths with a given number of peaks by number of peak plateaus, where a peak plateau is a run of consecutive peaks that is immediately preceded by an upstep and followed by a downstep.Comment: 2 page

The 136th manifestation of C_n

We show bijectively that the Catalan number C_n counts Dyck (n+1)-paths in which the terminal descent is of even length and all other descents to ground level (if any) are of odd length.Comment: 3 pages, LaTe

An identity for the central binomial coefficient

We find the joint distribution of three simple statistics on lattice paths of n upsteps and n downsteps leading to a triple sum identity for the central binomial coefficient {2n}-choose-{n}. We explain why one of the constituent double sums counts the irreducible pairs of compositions considered by Bender et al., and we evaluate some of the other sums.Comment: 5 page

Flexagons yield a curious Catalan number identity

Hexaflexagons were popularized by the late Martin Gardner in his first Scientific American column in 1956. Oakley and Wisner showed that they can be represented abstractly by certain recursively defined permutations called pats, and deduced that they are counted by the Catalan numbers. Counting pats by number of descents yields the curious identity Sum[1/(2n-2k+1)binom{2n-2k+1}{k}binom{2k}{n-k},k=0..n] = C(n), where only the middle third of the summands are nonzero.Comment: 4 pages, shorter method, suggested by Ira Gessel; other (minor) improvement