# Triangular summation method

2020 Mathematics Subject Classification: *Primary:* 40C05 [MSN][ZBL]

A matrix summation method defined by a triangular matrix

$$A=\| a_{nk}\|,\quad n,k=1,2,\ldots,$$

that is, by a matrix for which $a_{nk}=0$ for $k>n$. A triangulation summation method is a special case of a row-finite summation method. A triangular matrix $A$ is called normal if $a_{nn}\neq0$ for all $n$. The transformation

$$\sigma_n=\sum_{k=1}^na_{nk}s_k$$

realized by a normal triangular matrix $A$ has an inverse:

$$s_n=\sum_{k=1}^na_{nk}^{-1}\sigma_k,$$

where $A^{-1}=\| a_{nk}^{-1}\|$ is the inverse of $A$. This fact simplifies the proof of a number of theorems for matrix summation methods determined by normal triangular matrices. Related to the triangular summation methods are, e.g., the Cesàro summation methods and the Voronoi summation method.

#### References

[Ba] | S.A. Baron, "Introduction to the theory of summability of series", Tartu (1966) (In Russian) |

[Co] | R.G. Cooke, "Infinite matrices and sequence spaces", Macmillan (1950) |

[Ha] | G.H. Hardy, "Divergent series", Clarendon Press (1949) |

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Triangular summation method.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Triangular_summation_method&oldid=25646