Can an invertible matrix have an eigenvalue equal to 0?

    No.
    A matrix is nonsingular (i.e. invertible) iff its determinant is nonzero.
    To prove this we note that to solve the eigenvalue equation
    ##Avecv = lambdavecv##
    we have that
    ##lambdavecv – Avecv = vec0##
    ##=> (lambdaI – A)vecv = vec0##
    and hence for a nontrivial solution
    ##|lambdaI – A| = 0##.
    Let ##A## be an ##NxxN## matrix. If we did have ##lambda = 0## then:
    ##|0*I – A| = 0##
    ##|-A| = 0##
    ##=> (-1)^n|A| = 0##
    Note that a matrix inverse can be defined as:
    ##A^(-1) = 1/|A| adj(A)##
    where ##|A|## is the determinant of ##A## and ##adj(A)## is the classical adjoint or the adjugate of ##A## (the transpose of the cofactor matrix).
    Clearly ##(-1)^(n) ne 0##. Thus the evaluation of the above yields ##0## iff ##|A| = 0## which would invalidate the expression for evaluating the inverse since ##1/0## is undefined.
    So if the determinant of ##A## is ##0## which is the consequence of setting ##lambda = 0## to solve an eigenvalue problem then the matrix is not invertible.

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