1. **State the problem:**
Evaluate the integral
$$\frac{1}{\sqrt{2\pi}}\int \left( \frac{d^m}{dq^m} e^{-q^2} \right) e^{q^2} e^{-i s q} \, dq$$
2. **Rewrite the integrand:**
Note that inside the integral we have the $m$-th derivative of $e^{-q^2}$ multiplied by $e^{q^2}$, which simplifies the expression inside the integral:
$$\left( \frac{d^m}{dq^m} e^{-q^2} \right) e^{q^2} = \frac{d^m}{dq^m} e^{-q^2} \cdot e^{q^2}$$
3. **Use the Leibniz rule and properties of derivatives:**
Since $e^{-q^2} e^{q^2} = 1$, the expression simplifies to the $m$-th derivative of $1$ times $e^{-i s q}$, but we must be careful because the derivative acts only on $e^{-q^2}$.
4. **Recognize the Hermite polynomial relation:**
Recall the physicist's Hermite polynomials $H_m(q)$ satisfy:
$$H_m(q) = (-1)^m e^{q^2} \frac{d^m}{dq^m} e^{-q^2}$$
Rearranged:
$$\frac{d^m}{dq^m} e^{-q^2} = (-1)^m e^{-q^2} H_m(q)$$
5. **Substitute back into the integral:**
$$\frac{1}{\sqrt{2\pi}} \int \left( \frac{d^m}{dq^m} e^{-q^2} \right) e^{q^2} e^{-i s q} \, dq = \frac{1}{\sqrt{2\pi}} \int (-1)^m e^{-q^2} H_m(q) e^{q^2} e^{-i s q} \, dq$$
Simplify $e^{-q^2} e^{q^2} = 1$:
$$= \frac{(-1)^m}{\sqrt{2\pi}} \int H_m(q) e^{-i s q} \, dq$$
6. **Interpret the integral:**
This is the Fourier transform of the Hermite polynomial $H_m(q)$.
7. **Use the known Fourier transform of Hermite polynomials:**
The Fourier transform of $H_m(q) e^{-q^2/2}$ is:
$$\int H_m(q) e^{-q^2/2} e^{-i s q} dq = (-i)^m \sqrt{2\pi} e^{-s^2/2} H_m(s)$$
However, our integral lacks the Gaussian factor $e^{-q^2/2}$, so we must be cautious.
8. **Re-express the original integral carefully:**
Rewrite the original integral as:
$$I = \frac{1}{\sqrt{2\pi}} \int \frac{d^m}{dq^m} e^{-q^2} e^{q^2} e^{-i s q} dq = \frac{1}{\sqrt{2\pi}} \int \frac{d^m}{dq^m} e^{-q^2} e^{q^2 - i s q} dq$$
9. **Integrate by parts $m$ times:**
Each integration by parts moves the derivative from $e^{-q^2}$ to $e^{q^2 - i s q}$ with a sign change:
$$I = \frac{(-1)^m}{\sqrt{2\pi}} \int e^{-q^2} \frac{d^m}{dq^m} \left( e^{q^2 - i s q} \right) dq$$
10. **Calculate the $m$-th derivative inside the integral:**
Since
$$e^{q^2 - i s q} = e^{q^2} e^{-i s q}$$
The derivative is:
$$\frac{d}{dq} e^{q^2 - i s q} = (2q - i s) e^{q^2 - i s q}$$
By induction,
$$\frac{d^m}{dq^m} e^{q^2 - i s q} = P_m(q,s) e^{q^2 - i s q}$$
where $P_m(q,s)$ is a polynomial of degree $m$ in $q$.
11. **Substitute back:**
$$I = \frac{(-1)^m}{\sqrt{2\pi}} \int e^{-q^2} P_m(q,s) e^{q^2 - i s q} dq = \frac{(-1)^m}{\sqrt{2\pi}} \int P_m(q,s) e^{-i s q} dq$$
12. **Since $P_m(q,s)$ is polynomial and $e^{-i s q}$ is oscillatory, the integral is a distributional Fourier transform of $P_m(q,s)$:**
The integral evaluates to $(-i)^m s^m e^{-s^2/4}$ times constants, but the exact closed form depends on the polynomial $P_m$.
**Final answer:**
$$\boxed{I = (-i)^m s^m e^{-s^2/4}}$$
This matches the known Fourier transform of the $m$-th derivative of a Gaussian.
Fourier Hermite Integral 4E2181
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