EN
We study the existence of positive solutions of the nonlinear fourth order problem
$u^{(4)}(x) = λa(x)f(u(x))$,
u(0) = u'(0) = u''(1) = u'''(1) = 0,
where a: [0,1] → ℝ may change sign, f(0) < 0, and λ < 0 is sufficiently small. Our approach is based on the Leray-Schauder fixed point theorem.