arXiv subjects

David Callan

Publications and source records attributed to David Callan.

67 records · Page 4Linked to original sources

arXiv2005

A combinatorial interpretation of the eigensequence for composition

The monic sequence that shifts left under convolution with itself is the Catalan numbers with 130+ combinatorial interpretations. Here we establish a combinatorial interpretation for the monic sequence that shifts left under composition: it counts permutations that contain a 3241 pattern only as part of a 35241 pattern. We give two recurrences, the first allowing relatively fast computation, the second similar to one for the Catalan numbers. Among the 4 times 4! = 96 similarly restricted patterns involving 4 letters (such as 4\underline{2}31: a 431 pattern only occurs as part of a 4231), four different counting sequences arise: 64 give the Catalan numbers, 16 give the Bell numbers, 12 give sequence A051295 in OEIS, and 4 give a new sequence with an explicit formula.

math.CO↗

arXiv2005

Some identities for the Catalan and Fine numbers

We establish combinatorial interpretations of several identities for the Catalan and Fine numbers and, along the way, we present some new bijections of independent interest. Briefly, we show that C_{n} = 1/(n+1) Sum_{k} (n+1)choose(2k+1) (n+k)choose(k) counts ordered trees on n edges by number of interior vertices adjacent to a leaf, and C_{n} = 2/(n+1) Sum_{k} (n+1)choose(k+2) (n-2)choose(k) counts Dyck n-paths by number of long interior inclines. We also give an analogue for the Fine numbers of Touchard's Catalan number identity.

math.CO↗

arXiv2004

A combinatorial interpretation for a super-Catalan recurrence

Nicholas Pippenger and Kristin Schleich have recently given a combinatorial interpretation for the second-order super-Catalan numbers (u_{n})_{n>=0}=(3,2,3,6,14,36,...): they count "aligned cubic trees" on n internal vertices. Here we give a combinatorial interpretation of the recurrence u_{n} = Sum_{k=0}^{n/2-1} ({n-2}choose{2k} 2^{n-2-2k} u_{k}): it counts these trees by number of deep interior vertices where deep interior means "neither a leaf nor adjacent to a leaf".

math.CO↗

arXiv2004

Some bijections for restricted Motzkin paths

We give several bijections among restricted Motzkin paths, explaining why various parameters on these paths are equidistributed. For example, the number of doublerise-free Motzkin paths of length n is the same as the number of peak-free Motzkin paths of length n+1 and the parameter "number of doublefalls" has the same distribution on the former set as "number of valleys" does on the latter. The bijections are most easily presented recursively but we also give explicit descriptions using the notion of Motzkin tree.

math.CO↗

arXiv2004

Two Bijections for Dyck Path Parameters

Here we give two bijections, one to show that the number of UUU-free Dyck n-paths is the Motzkin number M_n, the other to obtain the (known) distributions of the parameters "number of UDUs" and "number of DDUs" on Dyck n-paths. The first bijection is straightforward, the second not quite so obvious.

math.CO↗

arXiv2004

A combinatorial proof of Sun's "curious" identity

A binomial coefficient identity due to Zhi-Wei Sun is the subject of half a dozen recent papers that prove it by various analytic techniques and establish a generalization. Here we give a simple proof that uses weight-reversing involutions on suitable configurations involving dominos and colorings. With somewhat more work, the method extends to the generalization also.

math.CO↗

arXiv2003

A combinatorial derivation of the number of labeled forests

Lajos Takacs gave a somewhat formidable alternating sum formula for the number of forests of unrooted trees on $n$ labeled vertices. Here we use a weight-reversing involution on suitable tree configurations to give a combinatorial derivation of Takacs' formula.

math.CO↗

arXiv2003

A uniformly distributed parameter on a class of lattice paths

Let G_n denote the set of lattice paths from (0,0) to (n,n) with steps of the form (i,j) where i and j are nonnegative integers, not both 0. Let D_n denote the set of paths in G_n with steps restricted to (1,0), (0,1), (1,1), so-called Delannoy paths. Stanley has shown that | G_n | = 2^(n-1) | D_n | and Sulanke has given a bijective proof. Here we give a simple parameter on G_n that is uniformly distributed over the 2^(n-1) subsets of [n-1] = {1,2,...,n-1} and takes the value [n-1] precisely on the Delannoy paths.

math.CO↗

arXiv2003

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.

math.CO↗

arXiv2002

A recursive bijective approach to counting permutations containing 3-letter patterns

We present a method, illustrated by several examples, to find explicit counts of permutations containing a given multiset of three letter patterns. The method is recursive, depending on bijections to reduce to the case of a smaller multiset, and involves a consideration of separate cases according to how the patterns overlap. Specifically, we use the method (i) to provide combinatorial proofs of Bona's formula {2n-3}choose{n-3} for the number of n-permutations containing one 132 pattern and Noonan's formula 3/n {2n}choose{n+3} for one 123 pattern, (ii) to express the number of n-permutations containing exactly k 123 patterns in terms of ballot numbers for k<=4, and (iii) to express the number of 123-avoiding n-permutations containing exactly k 132 patterns as a linear combination of powers of 2, also for k<=4. The results strengthen the conjecture that the counts are algebraic for all k.

math.CO↗

arXiv2002

Pattern avoidance in circular permutations

Circular permutations on {1,2,...,n} that avoid a given pattern correspond to ordinary (linear) permutations that end with n and avoid all cyclic rotations of the pattern. Three letter patterns are all but unavoidable in circular permutations and here we give explicit formulas for the number of circular permutations that avoid one four letter pattern. In the three essentially distinct cases, the counts are as follows: the Fibonacci number F_{2n-3} for the pattern 1324, 2^{n-1}-(n-1) for 1342, and 2^{n}+1-2n-{n}choose{3} for 1234.

math.CO↗

arXiv2002

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.

math.CO↗

arXiv2000

The eigenvectors of the right-justified Pascal triangle

We find the eigenvalues and eigenvectors of the n by n matrix with (i,j) entry \binom(i-1,n-j), establishing a conjecture of Peele and Stanica. Curiously, the eigenvectors can be chosen to form a matrix which is its own inverse.

math.CO↗