SymEigenValues3by3

Calculate the eigenvalues of 3 by 3 matrix.

܀ Interface

INTERFACE
  MODULE PURE FUNCTION SymEigenValues3by3(mat) RESULT(ans)
    REAL(DFP), INTENT(IN) :: mat(3, 3)
    REAL(DFP) :: ans(3)
  END FUNCTION SymEigenValues3by3
END INTERFACE

️܀ See example

$$\left(\begin{array}{ccc}-2 & -4 & 2 \\-4 & 1 & 2\\2 & 2 & 5\end{array}\right)$$

PROGRAM main
USE easifemBase

    REAL( DFP ) :: mat(3,3), w(3), exact(3)
    REAL( DFP ), PARAMETER :: tol= 1.0E-5

Test-1

    exact = [-5.51082, 3.65928, 5.85154]
    mat = RESHAPE([-2._DFP,-4._DFP,2._DFP,-4._DFP,1._DFP,2._DFP,2._DFP,2._DFP,5._DFP], [3,3])
    !!
    w = SymEigenValues(mat)
    CALL Display(w, "w=")
    !!
    CALL OK( ALL(SOFTEQ(w, exact, tol)), "")

END PROGRAM main

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