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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 …
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
$$
\left( \begin{ar …
6
votes
Fourier cosine transform from Erdélyi's Tables of Integral Transforms
According to the Table Errata reported in Mathematics of Computation, vol. 65, no. 215, 1996, pp. 1379–1386, this entry in Erdélyi's Tables of Integral Transforms is flawed. The exponent in the denomi …
3
votes
Accepted
Integrals involving fractions of exponentials
We can obtain power series in $a$ by expanding in geometric series. Since the specific treatment depends on the integration region, let's introduce lower and upper limits, $y_0 <y$,
$$
I = \int_{y_0 } …
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
$$
b-2 = b^2 \ …
2
votes
Hypergeometric function with changed argument
The relationship can be written using the dilation operator as
$$
_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)
$$
3
votes
Lower estimate of the minimal eigenvalue of a Hamiltonian
There is a generalization of the variational method already shown in
Linus Pauling's "Introduction to Quantum Mechanics" that isn't mentioned
much anymore (possibly because of limited practical use):
…
1
vote
How do I make the conceptual transition from multivariable calculus to differential forms?
I found B. Felsager's "Geometry, Particles and Fields" a useful introduction. It has a part I that presents field theory in the language most familiar in physics, and then a part II that puts things i …
1
vote
Accepted
Asymptotic for eigenvalues for the following ode?
I asked Mathematica about the boundary behavior. First, the $P^{1/4}_{\nu } $ solution: We have, for $\epsilon \searrow 0$,
$$
(\sin \epsilon )^{1/4} P^{1/4}_{\nu } (\cos \epsilon ) = \frac{2^{1/4} }{ …
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 …