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