10.1.3.2 Recurrence

Variables that are set in one iteration of a loop and used in a subsequent iteration introduce cross-iteration dependencies, or recurrences. Recurrence in a loop requires that the iterations to be executed in the proper order. For example:

   DO I=2,N
      A(I) = A(I-1)*B(I)+C(I)
   END DO

requires the value computed for A(I) in the previous iteration to be used (as A(I-1)) in the current iteration. To produce correct results, iteration I must complete before iteration I+1 can execute.