How to write an ordinary differential equation (in Python, using SymPy)

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Task

Differential equations are equations that contain differentials like $dy$ and $dx$, often in the form $\frac{dy}{dx}$. How can we write them using software?

Solution

This answer assumes you have imported SymPy as follows.

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from sympy import *                   # load all math functions
init_printing( use_latex='mathjax' )  # use pretty math output

The following code tells SymPy that $x$ is a variable and that $y$ is a function of $x$. It then expresses $\frac{dy}{dx}$ as the derivative of $y$ with respect to $x$.

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var( 'x' )                 # Let x be a variable.
y = Function('y')(x)       # Literally, y is a function, named y, based on x.
dydx = Derivative( y, x )  # How to write dy/dx.
dydx                       # Let's see how SymPy displays dy/dx.

${\displaystyle \frac{d}{dx}y\left(x\right)}$

Let’s now write a very simple differential equation, $\frac{dy}{dx}=y$.

As with how to do implicit differentiation, SymPy expects us to move everything to the left hand side of the equation. In this case, that makes the equation $\frac{dy}{dx}-y=0$, and we will use just the left-hand side to express our ODE.

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ode = dydx - y
ode

${\displaystyle -y\left(x\right)+\frac{d}{dx}y\left(x\right)}$

Content last modified on 24 July 2023.

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Contributed by Nathan Carter (ncarter@bentley.edu)