SymEigenValues2by2

Calculate the eigenvalues of 2 by 2 matrix.

܀ Interface

INTERFACE
  MODULE PURE FUNCTION SymEigenValues2by2(mat) RESULT(ans)
    REAL(DFP), INTENT(IN) :: mat(2, 2)
    REAL(DFP) :: ans(2)
  END FUNCTION SymEigenValues2by2
END INTERFACE

️܀ See example

PROGRAM main
USE easifemBase

    REAL( DFP ) :: mat(2,2), w(2), e, exact(2)
    REAL( DFP ), PARAMETER :: tol= 1.0E-12

Test 1

    e = 0.5
    exact = [1.0_DFP - e, 1.0_DFP + 4.0_DFP*e]
    mat(1,1) = 1+3*e
    mat(1,2) = 2*e
    mat(2,1) = 2*e
    mat(2,2) = 1
    !!
    w = SymEigenValues2by2(mat)
    !!
    CALL OK( ALL(SOFTEQ(w, exact, tol)), "e = "//tostring(e))

Test 2

    e = 0.01
    exact = [1.0_DFP - e, 1.0_DFP + 4.0_DFP*e]
    mat(1,1) = 1+3*e
    mat(1,2) = 2*e
    mat(2,1) = 2*e
    mat(2,2) = 1
    !!
    w = SymEigenValues2by2(mat)
    !!
    CALL OK( ALL(SOFTEQ(w, exact, tol)), "e = "//tostring(e))

END PROGRAM main

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