Oscillation criteria for fourth-order differential equations with middle term

Oscillation criteria for self-adjoint fourth-order differential equations (r(x)y")" - (q(x)y')' = p( x) y were established for various conditions on the coefficients r(x) > 0, q(x) and p(x). However, most of these results deal with the case when limx ->infinity integral(x)(1) q(s) ds < + infinity. In this note we give a new oscillation criterion in the case when this condition is not fulfilled, in particular when q(x) NE arrow + infinity(even with exponential growth). (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim