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Identities involving the tribonacci numbers squared via tilings with combs. (English) Zbl 1518.05013

Summary: The number of ways to tile an \(n\)-board (an \(n \times 1\) rectangular board) with \((\frac{1}{2}, \frac{1}{2}; 1)\)-, \((\frac{1}{2}, \frac{1}{2}; 2)\)-, and \((\frac{1}{2}, \frac{1}{2}; 2)\)-combs is \(T^2_{n+2}\), where \(T_n\) is the \(n\)-th tribonacci number. A \((\frac{1}{2}, \frac{1}{2}; m)\)-comb is a tile composed of \(m\) sub-tiles of dimensions \(\frac{1}{2}\times1\) (with the shorter sides always horizontal) separated by gaps of dimensions \(\frac{1}{2}\times1\). We use such tilings to obtain quick combinatorial proofs of identities relating the tribonacci numbers squared to one another, to other combinations of tribonacci numbers, and to the Fibonacci, Narayana’s cows, and Padovan numbers. Most of these identities appear to be new.

MSC:

05A19 Combinatorial identities, bijective combinatorics
05A10 Factorials, binomial coefficients, combinatorial functions
11B39 Fibonacci and Lucas numbers and polynomials and generalizations

Software:

OEIS

References:

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[10] OEIS Foundation Inc. (2022), The On-Line Encyclopedia of Integer Sequences, https://oeis.org. MSC2020: 05A19, 11B39
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