Correct

级数

/ jíshù / 
Strokes

Definition

(math.) series

Example

As a Noun

progression; series

1
Àn
按
jǐhéjíshù
几何级数
jiǎnshǎo
减少

decrease in a geometrical series

2
Děng
等
bǐ
比
  
/
děng
等
chājíshù
差级数

geometric/arithmetic progression

3
Jíshù
级数
biànhuàn
变换

transformation of series