3 n = 4m+3, arranged, 4m = 3 n-3.

According to the properties of geometric series,

2m=3*( 1-3^(n- 1))/( 1-3)=3+9+27+……+3^(n- 1)

Because 2m is an even number, every term of 3 n is an odd number, and (n- 1) must be an even number. Let n- 1=2k.

Then n=2k+ 1 where k is a positive integer,

The sequence {Cn} of the term * * * is a(2k+ 1).

Namely cn = 3 (2n+ 1)