MATH5175, 2023, Term 1 Assignment 2
Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: daixieit
MATH5175, 2023, Term 1
Assignment 2, 3th April
Due: 17th April
1. Consider a Lax pair of linear differential equations
Lψ = 0, λψ = M ψ, L = ∂g ∂y + u, M = ∂g(3) + a∂g , (1)
where u, a and ψ are functions of the independent variables x and y and λ constitutes an arbitrary constant. The coefficients u and a do not depend on λ .
a) It may be shown that the operator equation which guarantees the compatibility of the Lax pair (1) is given by
[L, M] = bL (2)
for a suitable function b. Determine the function b and derive the pair of equations for the functions u and a which guarantee that (2) is satisfied.
b) In the special case
ay = 3ug , ugg + au = 0,
eliminate a and show that the resulting third-order equation for u may be integrated once to obtain
ωgy = eu − f (y)e_2u, u = −eu , (3)
where f is a function of integration.
[Hint: Regard the third-order equation as a first-order linear ordinary differential equation for the quantity ωgy and use an integrating factor.]
c) In the case f (y) > 0, find an appropriate change of variables x\ = x, y\ = g(y), ω\ = ω + h(y)
such that (3) is transformed into the Tzitz´eica equation
ω\ y\ = eu\ − e_2u\ .
2. Consider the linear differential equation
ψgy + uψ = 0, (4)
where u and ψ are functions of the independent variables x and y .
a) Given two solutions ψ and ϕ of the linear equation (4), show that the compatibility condition which guarantees the existence of a potential S obeying the pair
Sg = ϕψg − ϕg ψ, Sy = ϕy ψ − ϕψy (5)
is satisfied. To what extent, in terms of constants/functions of integration, is S determined by the pair (5) for any fixed ψ and ϕ .
b) The linear equation (4) is known to be invariant under the Moutard transformation
ψ → ψ * = , u → u* = u + 2(ln ϕ)gy , (6)
where ϕ is another solution of (4) and S is defined by (5). On use of the pair (5), evaluate ψg(*) . Derive the transformation formula for u by evaluating ψg(*)y + u* ψ * = 0 with u* a priori unknown but only writing down the terms proportional to S which would occur if you did the complete calculation.
[Note: This is admissible since the validity of the transformation is postulated.] c) Show that the ansatz
u = u(z), ψ = e入(g_y)ψˆ(z), z = x + y (7)
reduces the linear equation (4) to the time-independent Schr¨odinger equation, where the arbitrary constant λ2 plays the role of the eigenvalue.
d) If we make the ansatz
ϕ = eu(g_y)(z) (8)
so that constitutes a solution of the time-independent Schr¨odinger equation corre- sponding to the eigenvalue µ2 and set
S = e(u+入)(g_y)(z)
then derive the pair of differential equations for implied by (5). Show, without differentiation or integration, that
= ψˆ2 − 2 ψˆ
µ + λ
is a necessary condition on . Verify that this is also sufficient. In view of the degrees of freedom considered in part a), what is the general solution of (5) in the special case (7), (8)?
e) Taking each ansatz in the cases c) and d) and using again
ψ * = e入(g_y)ψˆ* (z),
show that the invariance (6) reduces to the classical Darboux transformation for the time-independent Schr¨odinger equation (up to an irrelevant constant factor in ψ * ). Here, you are also required to express u* in terms of .
3. Consider the Hamiltonian H : ❘2 × ❘2 → ❘ given by
H(p, q) = p2 + f (q), q = |q|, (9)
where f is a differentiable function.
a) It is known that a transformation (p, q) → (P , Q) is canonical if and only if the canonical relations
{Qi , Qk } = 0, {Pi , Pk } = 0, {Qi , Pk } = δik
hold, where the Kronecker symbol is defined by δii = 1 and δik = 0 for i k . Given that
q1 = Q1 cos Q2 ,
q2 = Q1 sin Q2 ,
q1p1 + q2p2
q
P2 = q1p2 − q2p1
defines a canonical transformation, verify that
{Q1 , Q2 } = 0, {Q1 , P1 } = 1, {Q2 , P2 } = 1.
b) Determine p in terms of P and Q and formulate the Hamiltonian H(p, q) as a function (P , Q) of the new canonical coordinates.
c) Find an integral of motion F (P , Q) associated with a cyclic coordinate and explicitly parametrise in terms of Q1 and Q2 the level set
M(,F ) = {(P , Q) : (P , Q) = H0 , F (P , Q) = F0 }
for prescribed values H0 and F0 of the integrals of motion H and F . Is the Hamil- tonian system integrable? Give reasons for your answer.
d) Derive the Hamiltonian system in the canonical coordinates (P , Q). Show that Q1 satisfies the differential equation
Q˙1(2) = G(Q1 ), G(Q1 ) = 2H0 − 2f (Q1 ) −
and explicitly state the procedure which determines all canonical variables in terms of quadratures and inversions of functions.
e) If f (Q1 ) is such that there exists another functionally independent integral of motion (P , Q), can the integrals of motion H, F, be in involution? How can one exploit in connection with the trajectories of the Hamiltonian system in phase space with coordinates (P , Q)?
f) Explain why if H is of the form
H(p, q) = H1 (p1 , q1 ) + H2 (p2 , q2 )
then both H1 and H2 are integrals of motion.
g) This is a bonus question which allows you to make up for marks that you may have lost in the previous questions so that you may still get full marks.
Provide two non-constant functions f (q) for which three functionally independent in- tegrals of motion Fi (p, q) exist. Give reasons for your answer. No explicit calculations are necessary.
2023-04-19