In mathematics and computing, a triangular array of numbers, polynomials, or the like, is a doubly indexed sequence in which each row is only as long as the row's own index.
Notable particular examples include these:
- The Bell triangle, whose numbers count the partitions of a set in which a given element is the largest singleton
- Catalan's triangle, which counts strings of parentheses in which no close parenthesis is unmatched
- Euler's triangle, which counts permutations with a given number of ascents
- Floyd's triangle, whose entries are all of the integers in order
- Hosoya's triangle, based on the Fibonacci numbers
- Lozanić's triangle, used in the mathematics of chemical compounds
- Narayana triangle, counting strings of balanced parentheses with a given number of distinct nestings
- Pascal's triangle, whose entries are the binomial coefficients
Triangular arrays of integers in which each row is symmetric and begins and ends with 1 are sometimes called generalized Pascal triangles; examples include Pascal's triangle, the Narayana numbers, and the triangle of Eulerian numbers.
Arrays in which the length of each row grows as a linear function of the row number (rather than being equal to the row number) have also been considered.
Apart from the representation of triangular matrices, triangular arrays are used in several algorithms. One example is the CYK algorithm for parsing context-free grammars, an example of dynamic programming.
- Triangular number, the number of entries in such an array up to some particular row
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