used as combining form of L. triangulum TRIANGLE, in triangulo-pyramidal, triangulo-triangular adjs., applied to certain series of figurate numbers: see quots., and cf. PYRAMIDAL 4, TRIANGULAR 2 b.
[1646. F. Vieta, Opera, 294. In prima adfectione per unitatis crementum, in secunda per numeros triangulos, in tertia per numeros pyramidales, in quarta per numeros triangulo-triangulos, in quinta per numeros triangulo-pyramidales.]
1715. Phil. Trans., XXIX. 183. A series of Fractions … whose Numerator is a given Number and Denominators are triangular or pyramidal or triangulo-triangular Numbers, &c.
1785. Hutton, Math. Tables, 7. The several orders of figurate numbers, which he [Vieta] calls triangular, pyramidal, triangulo-triangular, triangulo-pyramidal.
