Error, (in dsolve/numeric/process_input) input system must be an ODE system, got independent variables {x, y} - Maple Help

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Error, (in ...) input system must be an ODE system, got independent variables ...

 Description The error message occurs only when there are unknown functions operating on different independent variables in the same problem.

Examples

Example 1
$g\left(y\right)$ entered instead of $g\left(x\right)$

 > $\mathrm{dsolve}\left(\left\{\mathrm{diff}\left(f\left(x\right),x\right)=g\left(y\right)\right\},\mathrm{numeric}\right);\phantom{\rule[-0.0ex]{0.0em}{0.0ex}}$

Example 2
Two separate ODE problems entered as one problem, each using a different independent variable.

 >

Solution
Split the problem into 2 parts:

 >
 ${\mathbf{proc}}\left({\mathrm{x_rkf45}}\right)\phantom{\rule[-0.0ex]{0.5em}{0.0ex}}{...}\phantom{\rule[-0.0ex]{0.5em}{0.0ex}}{\mathbf{end proc}}{,}{\mathbf{proc}}\left({\mathrm{x_rkf45}}\right)\phantom{\rule[-0.0ex]{0.5em}{0.0ex}}{...}\phantom{\rule[-0.0ex]{0.5em}{0.0ex}}{\mathbf{end proc}}$ (2.1)

Or to fix $'g'$ using:

 >
 ${\mathbf{proc}}\left({\mathrm{x_rkf45}}\right)\phantom{\rule[-0.0ex]{0.5em}{0.0ex}}{...}\phantom{\rule[-0.0ex]{0.5em}{0.0ex}}{\mathbf{end proc}}$ (2.2)
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