MA3AM/MA4AM ASYMPTOTIC METHODS
PROBLEM SHEET 6
Use Laplace’s method to derive the following asymptotic expansions.
7. Consider the general Laplace integral
where we suppose that h' (a) = h'' (a) = 0, h''' (a) < 0, and h' (t) < 0 for t ∈ (a, b], so that h(t) is maximised at the point of inflexion t = a. Show that
where f(t) ∼ f0(t − a)λ as t → a, with λ > −1 and f0 a constant.
8. (FOR STUDENTS TAKING MA4AM. NOT FOR STUDENTS TAKING MA3AM.)
Derive the two-term asymptotic expansion
as x → ∞, as follows. First show that h(t) = − sin2 t is maximised at t = 0, determine an expa代 写MA3AM/MA4AM ASYMPTOTIC METHODS PROBLEM SHEET 6
nsion of h(t) around this point up to terms of O(t4), and hence show that
Then Taylor expand the quartic term up to terms of O(t4) (valid since 0 < t < δ ≪ 1) to yield
Finally, expand the range of integration to ([0,∞) (incurring only exponentially small errors), and then proceed in the usual way. . .
9. (FOR STUDENTS TAKING MA4AM. NOT FOR STUDENTS TAKING MA3AM.)
Use Laplace’s method to show that
as x → ∞.
10. (FOR STUDENTS TAKING MA4AM. NOT FOR STUDENTS TAKING MA3AM.)
Use Laplace’s method to show that
as x → ∞.
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