Engler — Figurate Numbers. 



53 



For d=3. 



Table 28. 

 which is identical with Table 19. 



The set of series included in Table 21 is of peculiar inter- 

 est because it contains the familiar sets of series known 

 as polygonal numhers and pyramidal numbers. 



Polygonal numbers. These are the numbers in the sec- 

 ond row of Table 21, for (/=successively 1, 2, 3, 4 . . . . 



They are called polygonal because the numbers in the 

 series correspond to the numbers of points or dots which 



« See "Ueber polydimensionalen Zahlenfiguren ", by Th. Harmuth, Archiv 

 der Mathematik und Physik, Series I, Vol. LXIX, pp. 90-107. 



