For a complete specification of the routine see the NAG Fortran Library Manual. It also reports values of the eigenfunction and its derivatives.
Provision is made for discontinuities in the coefficient functions or their derivatives.
Section 3 of the routine document in the NAG Fortran Library Manual states conditions which p and q must satisfy. Examples are given in Sections 8.5 and 9 of the routine document in the NAG Fortran Library Manual. The true error is rarely more than twice, or less than a tenth, of the estimated error. See Section D for further details. If it is zero the routine generates a maximum step size internally.