WHEN IS A POWER-SERIES RING N-ROOT CLOSED

Given commutative rings A subset of or equal to B, we present a necess ary and sufficient condition for the power series ring A[[X]] to be n- root closed in B[[X]]. This result leads to a criterion for the the po wer series ring A[[X]] over an integral domain A to be n-root closed ( in its quotient field). For a domain A, we prove: if A is Mori (for ex ample, Noetherian), then A[[X]] is n-root closed iff A is n-root close d; if A is Prufer, then A[[X]] is root closed iff A is completely inte grally closed.