Let p be a prime number, m = 2 a positive integer, and t a unit of R = Z_p^m, the ring of integers modulo p^m. Let
N = p^kn with gcd(p, n) = 1. In this work, we give a simple and short proof that the quotient ring R[X]I
is a principal ring. This allow us to study (1 + tp)-constacyclic codes of arbitrary length and give a characterization of self-dual (1 + tp) -constacyclic codes over Z_p^m.

Chain ring, Constacyclic codes, Dual codes, self-dual codes