invariants.for

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      SUBROUTINE invariants(A,INV1,INV2,NDI)
C>    1ST AND 2ND INVARIANTS OF A TENSOR
      IMPLICIT NONE
      include 'PARAM_UMAT.INC'
C
      INTEGER NDI,I1
      DOUBLE PRECISION A(ndi,ndi),AA(ndi,ndi)
      DOUBLE PRECISION INV1,INV1AA, INV2
C
      inv1=zero
      inv1aa=zero
      aa=matmul(a,a)
      DO i1=1,ndi
         inv1=inv1+a(i1,i1)
         inv1aa=inv1aa+aa(i1,i1)
      END DO
         inv2=(one/two)*(inv1*inv1-inv1aa)
C
      RETURN
      END SUBROUTINE invariants