pinvariants.for

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      SUBROUTINE pinvariants(A,INV4,NDI,ST,LAMBDA,BARLAMBDA,DET)
C>    AND 4TH PSEUDO-INVARIANTS OF A TENSOR
      IMPLICIT NONE
      include 'PARAM_UMAT.INC'
C
      INTEGER NDI,I,J
      DOUBLE PRECISION A(ndi,ndi),DET,SCALE1,ST(ndi,ndi),LAMBDA
      DOUBLE PRECISION BARLAMBDA,INV4
C
        inv4=zero
      DO i=1,ndi
        DO j=1, ndi
            inv4=inv4+a(i,j)*st(i,j)
        ENDDO
      ENDDO
C     STRETCH
      scale1=det**(-one /three)
      barlambda=dsqrt(inv4)
      lambda=barlambda/scale1
C
      RETURN
      END SUBROUTINE pinvariants