First term in this index = t1 = 2; next term respectively t2 = 4; t3 = 6; t4 = 8; ,
t2 - t↑ = 4 - 2 = 2 =
t3 - t2 = 6 - 4 = 2
t4 - t3 = 8 - 6 = 2
Thus, difference of two consecutive terms = d = 2
This general difference is a constant.
The given index is an arithmetic series.
Common difference = d = 2