Abstract
We study differential expressions of the fourth order and find sutficient conditions to ensure that they are not of the limit-circle type. In particular, we show that the differential expression y(4) + ry is never of the limit-circle type as long as r is not an unbounded oscillatory function; this partially answers an open question. Some of the results are deduced as consequences of new results on the nonlinear limit-point/limit-circle problem.
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