I've seen some problems where the OP writes integrals in this form
$$\int {dt} f\left( t \right)$$
or for double integrals
$$\int {dx} \int {dtf\left( {t,x} \right)} $$
Do they represent another kind of integrals, or is it just notation?
I've seen some problems where the OP writes integrals in this form
$$\int {dt} f\left( t \right)$$
or for double integrals
$$\int {dx} \int {dtf\left( {t,x} \right)} $$
Do they represent another kind of integrals, or is it just notation?
This is just notation. In general, $$\int f(t) dt = \int dt f(t)$$
In fact you can move the $dt$ term anywhere you want--as long as it remains within its corresponding integral. So $$\int dx \int dt f(t, x) = \int \left( \int f(t, x) dt \right) dx = \int \int f(t, x) dt dx.$$
However, $$\int dx\,dt \color{red}{\int f(t, x)} \neq \int \int f(t, x) \, dt \, dx$$
because on the LHS, the right integral in the red has no $d$ term and thus is nonsense.