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Special functions, orthogonal polynomials, harmonic analysis, ordinary differential equations (ODE's), differential relations, calculus of variations, approximations, expansions, asymptotics.

6 votes

Value of an integral

It seems your conjecture is true. Mathematica gives the result $$ (1 + 4^n (-1 + n) n \mbox{Beta} [1/2, -1 + n, 2 + n] - 4^n n (1 + n) \mbox{Beta} [1/2, 1 + n, n])/(2 (1 + n)) $$ in terms of the inc …
Michael Engelhardt's user avatar
8 votes
Accepted

Periodic eigenfunctions for 2D Dirac operator

$$ \left( \begin{array}{c} 1 \\ \frac{k_x - ik_y }{\sqrt{k_x^2 + k_y^2 } } \end{array} \right) e^{i(k_x x + k_y y)} \ \ \ \mbox{with eigenvalue} \ \ \ i\sqrt{k_x^2 + k_y^2 } $$ and $$ \left( \begin{ar …
Michael Engelhardt's user avatar
6 votes

Fourier cosine transform from Erdélyi's Tables of Integral Transforms

According to the Table Errata reported in Mathematics of Computation, vol. 65, no. 215, 1996, pp. 1379–1386, this entry in Erdélyi's Tables of Integral Transforms is flawed. The exponent in the denomi …
Michael Engelhardt's user avatar
3 votes
Accepted

Integrals involving fractions of exponentials

We can obtain power series in $a$ by expanding in geometric series. Since the specific treatment depends on the integration region, let's introduce lower and upper limits, $y_0 <y$, $$ I = \int_{y_0 } …
Michael Engelhardt's user avatar
23 votes

Is there a general solution for the differential equation $f''(x) = f(f(x))$?

The equation has solutions with powers, $f(x) = ax^b$. Inserting this ansatz, one has $$ a b (b-1) x^{b-2} = a (a x^b)^b = a^{b+1} x^{b^2} \ , $$ so the requirements on $a$ and $b$ are $$ b-2 = b^2 \ …
Michael Engelhardt's user avatar
2 votes

Hypergeometric function with changed argument

The relationship can be written using the dilation operator as $$ _2 F_1 (a,b,c,p\cdot z) = \exp \left( \ln p \cdot z \frac{\partial}{\partial z} \right)\, {}_2 F_1 (a,b,c,z) $$
Michael Engelhardt's user avatar
3 votes

Lower estimate of the minimal eigenvalue of a Hamiltonian

There is a generalization of the variational method already shown in Linus Pauling's "Introduction to Quantum Mechanics" that isn't mentioned much anymore (possibly because of limited practical use): …
Michael Engelhardt's user avatar
1 vote

How do I make the conceptual transition from multivariable calculus to differential forms?

I found B. Felsager's "Geometry, Particles and Fields" a useful introduction. It has a part I that presents field theory in the language most familiar in physics, and then a part II that puts things i …
1 vote
Accepted

Asymptotic for eigenvalues for the following ode?

I asked Mathematica about the boundary behavior. First, the $P^{1/4}_{\nu } $ solution: We have, for $\epsilon \searrow 0$, $$ (\sin \epsilon )^{1/4} P^{1/4}_{\nu } (\cos \epsilon ) = \frac{2^{1/4} }{ …
Michael Engelhardt's user avatar
3 votes
Accepted

On an integral equation

The answer is no. A counterexample is $$ f(t,x) = x - \frac{1}{2} -\frac{1}{24} t^2 $$ $$ B(t,s,x) = \left( x-\frac{1}{2} \right) (t-s) $$ (Method: I obtained this by expanding $f$ and $B$ into power …
Michael Engelhardt's user avatar