## What does this notation mean

1. The problem statement, all variables and given/known data

Here is the context:
suppose $f=(f_1,...,f_{n+1})$ is a smooth retraction of $B^{n+1}$ onto $S^n$

But what does the following statement mean?
$\int _{S^n}f_1df_2\wedge df_3 \wedge ... \wedge df_{n+1}$

2. Relevant equations

3. The attempt at a solution

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 this looks like it is being used in context of stokes theorem. any more information i think would be doing the problem. i think you can use the most obvious retract for this.