todai-math 2021 QII.1

todai-math · Japan · todai-engineering-math__paper1 Systems of differential equations
Consider that complex-valued functions $p(x)$ and $q(x)$ satisfy the simultaneous ordinary differential equations below: $$\frac{\mathrm{d}p(x)}{\mathrm{d}x} = -ib\, q(x)\exp(-2iax),$$ $$\frac{\mathrm{d}q(x)}{\mathrm{d}x} = -ib\, p(x)\exp(2iax).$$ Here, $i$ is the imaginary unit, and $a$ and $b$ are real constants.
Derive the simultaneous ordinary differential equations for complex-valued functions $f(x)$ and $g(x)$, based on the change of variables $f(x) = p(x)\exp(iax)$ and $g(x) = q(x)\exp(-iax)$.
Consider that complex-valued functions $p(x)$ and $q(x)$ satisfy the simultaneous ordinary differential equations below:
$$\frac{\mathrm{d}p(x)}{\mathrm{d}x} = -ib\, q(x)\exp(-2iax),$$
$$\frac{\mathrm{d}q(x)}{\mathrm{d}x} = -ib\, p(x)\exp(2iax).$$
Here, $i$ is the imaginary unit, and $a$ and $b$ are real constants.

Derive the simultaneous ordinary differential equations for complex-valued functions $f(x)$ and $g(x)$, based on the change of variables $f(x) = p(x)\exp(iax)$ and $g(x) = q(x)\exp(-iax)$.