# Triangular array

Not to be confused with Triangular matrix.
The triangular array whose right-hand diagonal sequence consists of Bell numbers

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.

## Examples

Notable particular examples include these:

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.[9]

## Generalizations

Triangular arrays may list mathematical values other than numbers; for instance the Bell polynomials form a triangular array in which each array entry is a polynomial.[10]

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.[11]

## Applications

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.[12]

Romberg's method can be used to estimate the value of a definite integral by completing the values in a triangle of numbers.[13]

The Boustrophedon transform uses a triangular array to transform one integer sequence into another.[14]

• Triangular number, the number of entries in such an array up to some particular row

## References

1. ^ Shallit, Jeffrey (1980), "A triangle for the Bell numbers", A collection of manuscripts related to the Fibonacci sequence, Santa Clara, Calif.: Fibonacci Association, pp. 69–71, MR 624091.
2. ^ Kitaev, Sergey; Liese, Jeffrey (2013), "Harmonic numbers, Catalan's triangle and mesh patterns", Discrete Mathematics 313 (14): 1515–1531, doi:10.1016/j.disc.2013.03.017, MR 3047390.
3. ^ Velleman, Daniel J.; Call, Gregory S. (1995), "Permutations and combination locks", Mathematics Magazine 68 (4): 243–253, doi:10.2307/2690567, MR 1363707.
4. ^ Miller, Philip L.; Miller, Lee W.; Jackson, Purvis M. (1987), Programming by design: a first course in structured programming, Wadsworth Pub. Co., pp. 211–212, ISBN 9780534082444.
5. ^ Hosoya, Haruo (1976), "Fibonacci triangle", The Fibonacci Quarterly 14 (2): 173–178.
6. ^ Losanitsch, S. M. (1897), "Die Isomerie-Arten bei den Homologen der Paraffin-Reihe", Chem. Ber. 30: 1917–1926, doi:10.1002/cber.189703002144.
7. ^ Barry, Paul (2011), "On a generalization of the Narayana triangle", Journal of Integer Sequences 14 (4): Article 11.4.5, 22, MR 2792161.
8. ^ Edwards, A. W. F. (2002), Pascal's Arithmetical Triangle: The Story of a Mathematical Idea, JHU Press, ISBN 9780801869464.
9. ^ Barry, P. (2006), "On integer-sequence-based constructions of generalized Pascal triangles", Journal of Integer Sequences 9 (06.2.4): 1–34.
10. ^ Rota Bulò, Samuel; Hancock, Edwin R.; Aziz, Furqan; Pelillo, Marcello (2012), "Efficient computation of Ihara coefficients using the Bell polynomial recursion", Linear Algebra and its Applications 436 (5): 1436–1441, doi:10.1016/j.laa.2011.08.017, MR 2890929.
11. ^ Fielder, Daniel C.; Alford, Cecil O. (1991), "Pascal's triangle: Top gun or just one of the gang?", in Bergum, Gerald E.; Philippou, Andreas N.; Horadam, A. F., Applications of Fibonacci Numbers (Proceedings of The Fourth International Conference on Fibonacci Numbers and Their Applications, Wake Forest University, N.C., U.S.A., July 30–August 3, 1990), Springer, pp. 77–90, ISBN 9780792313090.
12. ^ Indurkhya, Nitin; Damerau, Fred J., eds. (2010), Handbook of Natural Language Processing, Second Edition, CRC Press, p. 65, ISBN 9781420085938.
13. ^ Thacher, Jr., Henry C. (July 1964), "Remark on Algorithm 60: Romberg integration", Communications of the ACM 7 (7): 420–421, doi:10.1145/364520.364542.
14. ^ Millar, Jessica; Sloane, N. J. A.; Young, Neal E. (1996), "A new operation on sequences: the Boustrouphedon transform", Journal of Combinatorial Theory, Series A 76 (1): 44–54, arXiv:math.CO/0205218, doi:10.1006/jcta.1996.0087.