Weierstrass’ Elliptic Function Solution to the Autonomous Limit of the String Equation of Type (2,5)*


In this article, we study the string equation of type (2,5), which is derived from 2D gravity theory or the string theory. We consider the equation as a 4th order analogue of the first Painlevé equation, take the autonomous limit, and solve it concretely by use of the Weierstrass’ elliptic function.

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Sasaki, Y. (2014) Weierstrass’ Elliptic Function Solution to the Autonomous Limit of the String Equation of Type (2,5)*. Advances in Pure Mathematics, 4, 494-497. doi: 10.4236/apm.2014.48055.

1. Introduction

1.1. The String Equation of Type (2,5)

Put. Consider the commutator equation of ordinary differential operators

We call it the string equation (or Douglas equation) of type, which appears in the string theory or the theory of quantum gravity in 2D [1] -[9] . In the followings, we set,.

In the case where, , the string equation is written as an ODE satisfied by the potential w of Sturm-Liouville operator, and then, by a fractional linear transformation, it is reduced to the first Painlevé equation [10] [11]

, (PI)

which is equivalent to the Hamiltonian system:

In the case where, , yields


where, , are integral constants. By the fractional linear transformation, ,

and putting, the string equation is reduced to

, (S)

We also call it the string equation of type (2,5). Note that (S) coincides the 4th order equation of the first Painlevé hierarchy [12] -[15]

, (2nPI)

for, where is an expression of a given meromorphic function w defined by and.

1.2. Degenerated Garnier System

Equation (S) is also obtained as follows. Consider a 2D degenerated Garnier system [16] [17] :


which is a 2D analogue of (PI) in the theory of isomonodromic deformations. If we fix one of the independent variables, we get a Hamiltonian system with only one independent variable as follows:

From the above system, eliminating, , and putting, we obtain (S). So, Equation (S) is 4th order analogue of (PI) in the double sences.

It is already known by Shimomura [18] that every solution to (S) is meromorphic on, and that every pole of every solution is double one with its residue 0.

1.3. Autonomous Limit of the First Painlevé Equation

The first Painlevé equation (PI) has the autonomous limit [11] . Replacing by with a constant, and taking limit, we obtain which is solved by the Weierstrass’ elliptic function [10] [11] . The relation between the fundamental 2-form before and after the replacement is


1.4. Results

It is quite natural to think that:

Conjecture. Each equation of the first Painlevé hierarchy has the autonomous limit, and which is satisfied by the Weierstrass’ elliptic function.

For, the statement is valid, i.e.

Theorem A. Replacing by, or replacing by with a constant, and taking limit, we obtain the autonomous limit of the 4th order equation of the first Painlevé hierarchy (S). Moreover, the relation between the fundamental 2-form before and after the replacement is


It is easy to show the above. The autonomous limit is given by

, (A)

Theorem B. The autonomous limit Equation (A) has a solution concretely described by the Weierstrass’ elliptic function as



Remark. Modulus of the elliptic function is determined by the constants a and b. g2 and g3 in the elliptic function theory are as follows:


The next section is devoted to give the proof of Theorem B.

2. Proof of Theorem B

Put, i.e.

. (1)

Multiplying both sides of by, and integrating it, we obtain a first integral of (A)


In order to find the elliptic function solution, let w satisfy the relation:

. (3)

Substituting (3), and into (2), we have

So, if we take

then solutions of (3) satisfy (2). Now, in order to reduce to, we use the scale transformation,. Immediately we obtain, and also,.


*Dedicated to Professor Masafumi Yoshino on the occasion of his 60th birthday.

Conflicts of Interest

The authors declare no conflicts of interest.


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