Subsequent subsections treat the computation of Struve functions. The treatment
of Lommel and Anger–Weber functions is similar. For a review of methods for the computation of
see Zanovello (1975).
Although the power-series expansions (11.2.1) and (11.2.2),
and the Bessel-function expansions of §11.4(iv) converge for all
finite values of
, they are cumbersome to use when
is large owing to
slowness of convergence and cancellation. For large
and/or
the
asymptotic expansions given in §11.6 should be used instead.
For numerical purposes the most convenient of the representations given in
§11.5, at least for real variables, include the integrals
(11.5.2)–(11.5.5) for
and
. Subsequently
and
are obtainable via (11.2.5) and (11.2.6). Other integrals
that appear in §11.5(i) have highly oscillatory integrands unless
is small.
A comprehensive approach is to integrate the defining inhomogeneous differential equations (11.2.7) and (11.2.9) numerically, using methods described in §3.7. To insure stability the integration path must be chosen so that as we proceed along it the wanted solution grows in magnitude at least as rapidly as the complementary solutions.
Suppose
and
is real and positive. Then from the limiting forms
for small argument (§§11.2(i), 10.7(i),
10.30(i)), limiting forms for large argument
(§§11.6(i), 10.7(ii), 10.30(ii)), and the
connection formulas (11.2.5) and (11.2.6), it is seen that
and
can be computed in a stable manner
by integrating forwards, that is, from the origin toward infinity. The solution
needs to be integrated backwards for small
, and either
forwards or backwards for large
depending whether or not
exceeds
. For
both forward and backward integration
are unstable, and boundary-value methods are required (§3.7(iii)).
Sequences of values of
and
, with
fixed, can be computed by application of the inhomogeneous difference equations
(11.4.23) and (11.4.25). There are similar problems to those
described in §11.13(iv) concerning stability. In consequence
forward recurrence, backward recurrence, or boundary-value methods may be
necessary. See §3.6 for implementation of these methods, and with
the Weber function
as an example.