# Is $D^4\cup_\phi D^3\times S^1$ a manifold

Let $M:=D^4\cup_\phi D^3\times S^1$, where $\phi$ is the smashing product, i.e.$\phi:S^2\times S^1\overset{\wedge}\to S^3$.

Q:

• Is $M$ a manifold, since $\phi$ is not a smooth.

• Can we find a smooth map $f:S^2\times S^1\to S^3$ which is homotopic to $\phi$.