# change of variables question in 1st order proof

• Sep 5th 2013, 10:06 PM
mathlover10
change of variables question in 1st order proof
$\displaystyle \frac{dy}{dt}F(y(t))=g(t),\int_{to}^{t}\frac{dy}{d t}F(y(t))=\int_{to}^{t}g(t)=F(y(t))-F(yo), \int_{t}^{to}f(r)dr=\int_{to}^{t}g(s)ds,$
in differential equations when solving an initial value problem for c, you have this proof in Braun where he changes f(t) to f(r), g(t) to g(s)- I'm not sure what the reasoning is, maybe you can use any arbitrary variable? the time dependence of y on t also remains?
• Sep 5th 2013, 11:13 PM
chiro
Re: change of variables question in 1st order proof
Hey mathlover10.

They are just dummy variables: my guess is that he is trying to avoid using the variables t, t0, y and y0 for the limits.

Remember that Integral 0 to x f(x)dx is a lot more complicated than Integral 0 to x f(s)ds. The second one is usually what is meant but sometimes you can get situations resembling the first (The Fresnel integrals come to mind).