Suspension of a product - tricky homotopy equivalence Let $(X,x_0), (Y,y_0)$ be well-pointed spaces (inclusion of the basepoints is a cofibration). Show the following homotopy equivalence
$$
\Sigma (X\times Y) \simeq \Sigma X \lor \Sigma Y \lor \Sigma (X\land Y),
$$
where $\Sigma$ means a suspension (or reduced suspension if one prefers since it doesn't matter for well-pointed spaces) and $\land$ is a smash product.
Assuming we are using reduced suspension, it is quite clear that $\Sigma X \lor \Sigma Y$ is a subspace of the lhs, but I don't know how to somehow pull it outside and get $\Sigma (X\land Y)$.
Edit: I know the homotopy equivalence can be deduced from general theorems on "homotopy functors" (I hope that's how they are called in English) from chapter 7.7 in Spanier. But I was told there is an explicit proof and that's the one I'm looking for.
 A: I don't know how explicit we can go, but I'll give it a try. We have to go first through the homotopy-theoretical part. 
Since $\{ * \} \subseteq X, \{ * \} \subseteq Y$ are cofibrations, $X \vee Y \subseteq X \times Y$ also is. Let $Z$ be a pointed space and consider the long exact sequence of homotopy for the pair $X \vee Y \subseteq X \times Y$, ie. the sequence
$\ldots \rightarrow [\Sigma ^{2}(X \vee Y), Z] \rightarrow [\Sigma(X \wedge Y), Z] \rightarrow [\Sigma(X \times Y), Z] \rightarrow [\Sigma(X \vee Y), Z] \rightarrow [X \wedge Y, Z] \rightarrow \ldots$,
where $[-,-]$ is the pointed set of homotopy classes of basepoint-preserving maps. Note that for any $n \geq 0$, $\Sigma^{n}(X \vee Y)$ is homeomorphic to $\Sigma^{n}X \vee \Sigma ^{n} Y$. I will not distinguish between the two. 
Let $k \geq 1$ and define a map 
$\psi ^{k}: \Sigma^{k}(X \times Y) \rightarrow \Sigma^{k}X \vee \Sigma^{k}Y$
$\psi ^{k} = \Sigma^{k}(i_{X} \pi_{X}) + \Sigma^{k}(i_{Y} \pi_{Y})$, 
where $\pi: X \times Y \rightarrow X, Y$ are the projections and $i: X, Y \rightarrow X \vee Y$ are the inclusions. Addition is performed via the suspension structure on $\Sigma^{k}(X \times Y)$, so this is why we require $k \geq 1$. (Observe that even though I denote it by addition this is not necessarily commutative for $k=1$.)
If $j: X \vee Y \hookrightarrow X \times Y$ is the inclusion, then I claim that $\psi ^{k}$ is the left inverse to $\Sigma^{k}j$, ie. $\psi ^{k} \circ \Sigma^{k}j = id_{\Sigma^{k}(X \vee Y)}$. This is important because $\Sigma^{k}j$ are connecting maps in the long exact sequence of homotopy. Indeed, one computes
$\psi ^{k} \circ (\Sigma^{k}j) = (\Sigma^{k}(i_{X} \pi_{X}) + \Sigma^{k}(i_{Y} \pi_{Y})) \circ \Sigma^{k}j = \Sigma^{k}(i_{X} \pi_{X} j) + \Sigma^{k}(i_{Y} \pi _{Y} j)  = \Sigma^{k}(id_{X} \vee const) + \Sigma^{k}(const \vee id_{Y}) \simeq (\Sigma^{k}id_{X} + const) \vee (const + \Sigma^{k}id_{Y}) \simeq \Sigma^{k}id_{X} \vee \Sigma^{k}id_{Y} \simeq id_{\Sigma^{k}X \vee \Sigma^{k}Y}$.
(One can also see this geometrically.) This immediately implies that for all $k \geq 1$ and all $Z$ the $[\Sigma^{k}(X \times Y), Z] \rightarrow [\Sigma^{k}(X \vee Y), Z]$ induced by $j$ is surjective and - by exactness of the long exact sequence - that for all $n \geq 1$ the map $[\Sigma^{n}(X \smash Y), Z] \rightarrow [\Sigma^{n}(X \times Y), Z]$ has zero kernel. In particular, for $k=1$ we have the short exact sequence of groups 
$0 \rightarrow [\Sigma(X \wedge Y), Z] \rightarrow [\Sigma(X \times Y), Z] \rightarrow [\Sigma(X) \vee \Sigma(Y), Z] \rightarrow 0$
Moreover, the map induced by $\psi^{1}$ splits it and shows that there is a natural isomorphism
$\phi: [\Sigma(X \wedge Y), Z] \rtimes [\Sigma(X) \vee \Sigma(Y), Z] \rightarrow [\Sigma(X \times Y), Z]$,
of groups, where the product is only semi-direct, because our groups are not necessarily abelian. This is enough for our purposes, since we also have natural bijections
$[\Sigma(X \wedge Y), Z] \rtimes [\Sigma(X) \vee \Sigma(Y), Z] \simeq [\Sigma(X \wedge Y), Z] \times [\Sigma(X) \vee \Sigma(Y), Z] \simeq [\Sigma(X \wedge Y) \vee \Sigma(X) \vee \Sigma(Y), Z]$.
(The second one follows from from the fact that $\vee$ is the direct sum in the category of pointed spaces.) Yoneda lemma establishes that there is an isomorphism 
$\theta: \Sigma(X \times Y) \rightarrow _{\simeq} \Sigma(X \smash Y) \vee \Sigma(X) \vee \Sigma(Y) $
in the homotopy category of pointed spaces, ie. a homotopy equivalence that we were after. It takes a little bookkeeping in the above Yoneda-lemma argumentation to see that such map is given by 
$\theta = \Sigma(p) + \psi^{i} = \Sigma(p) + \Sigma^{1}(i_{X} \pi_{X}) + \Sigma^{1}(i_{Y} \pi_{Y})$, 
where $p: X \times Y \rightarrow X \wedge Y$ is the natural projection. (This is what we get if we start with $id \in [\Sigma(X \wedge Y) \vee \Sigma(X) \vee \Sigma(Y), \Sigma(X \wedge Y) \vee \Sigma(X) \vee \Sigma(Y)]$ and trace it back by all the bijections above to $[\Sigma(X \times Y), \Sigma(X \wedge Y) \vee \Sigma(X) \vee \Sigma(Y)]$ - and this is the way to discover the isomorphisms "hidden" by Yoneda lemma.) 
I understand that my exposition is far from perfect, but if you would like me to go into more detail over some parts, please comment. 
A: This is proposition 4I.1 in Hatcher. The argument given there is geometric and elementary. It goes as follows:
We assume $X$ and $Y$ are CW complexes and we use the reduced suspension everywhere (which in this case is homotopy equivalent to the free suspension). I assume that your more general situation of well-pointed spaces works the same.
Consider the reduced Join $X*Y$ which is, like the regular join, the quotient of $X\times Y\times [0,1]$ by the relations 
$$
(x,y_1,0)\sim(x,y_2,0)
$$
$$
(x_1,y,1)\sim(x_2,y,1)
$$
but we also collapse to a point the segment $(x_0,y_0,t)$. i.e. we collapse one "face" of the "cube" to $X$ and the opposite to $Y$ and the segment connecting the base points. Now, we glue in the cones on $X$ and $Y$ respectively along the collapsed faces and obtain a space
$$
Z=CX\sqcup_X (X*Y) \sqcup_Y CY
$$
We will show that it is homotopy equivalent to the two spaces in your question. 
On the one hand, if we collapse the two cones $CX$ and $CY$, what we get (by unwinding the definitions) is precisely the reduced suspension of $X\times Y$ (note that we used the reduced join). Since collapsing a contractible subcomplex does not change the homotopy type, we have a homotopy equivalence $Z\simeq \Sigma (X\times Y)$. 
On the other hand, inside the reduced join $X*Y$, we have the spaces  $X*y_0$ and $x_0*Y$ which are also contractible, as they are just cones on $X$ and $Y$ respectively, and collapsing them gives $\Sigma(X)\vee \Sigma(X\wedge Y)\vee \Sigma(Y)$. As before, this space is homotopy equivalent to $Z$ and we are done.
A: Here is a very simple proof that works in any $(\infty,1)$-category with suitable (co)limits. If you don't want to care about what exactly this is just imagine a nice category of spaces like simplicial sets or CW-complexes.
Let $X,Y$ be pointed spaces, observe the following diagram with the obvious maps:
$$\require{AMScd}
\begin{CD}
X \times Y @>{\operatorname{pr}_2}>> Y @>>> 1\\
@V{\operatorname{pr}_1}VV @VVV  @VVV \\
X @>>> \Sigma (X \wedge Y) @>>> \Sigma Y \vee \Sigma (X \wedge Y) \\
@VVV @VVV @VVV \\
1 @>>> \Sigma X \vee \Sigma(X \wedge Y) @>>> \Sigma X \vee \Sigma Y \vee \Sigma(X\wedge Y)
\end{CD}$$
Now observe that the top left square is a pushout square. You can think of this as the suspension of a smash is the join.
It's not difficult to see that the bottom left square and the top right square are also pushouts.
Note that from these facts we can compose 2 of the 3 squares and get a characterization of the cofiber of $\operatorname{pr}_1$ or $\operatorname{pr}_2$.
Finally the bottom right square is also a pushout by descent. Let the bottom right square be the top face of a cube. We give the cube a bottom face with the following pushout square:
$$\require{AMScd}
\begin{CD}
1 @>>> \Sigma Y \\
@VVV @VVV \\
\Sigma X @>>> \Sigma X \vee \Sigma Y
\end{CD}
$$
It's not difficult to check that all the other faces are pullbacks hence our top square must be a pushout.
By composing these 4 pushout squares you have that
$$\Sigma (X \times Y) \simeq \Sigma X \vee \Sigma Y \vee \Sigma(X\wedge Y)$$
References


*

*The James and Hilton-Milnor Splittings, & the metastable EHP sequence
arXiv:1912.04130.

