Edited to fix the example, as per Zack's suggestion.

Edit 2: So it turns out that when I think 'manifold' I tend to assume the nicest possible object. As I believe is standard, I would like to assume that all manifolds are 2nd countable and Hausdorff. Furthermore, let's say that our manifolds are connected and closed.

The Whitney embedding theorem states that any smooth $n$-manifold may be smoothly embedded into $\mathbb{R}^{2n}$. If we consider embeddings into more general $k$-dimensional manifolds, is it possible find a 'universal' $n$-universal' manifold of dimension less than $2n$?

For example, a non-orientable 2-manifold cannot be embedded into $\mathbb{R}^3$, demonstrating the sharpness of the Whitney embedding theorem.

However, there are 3-manifolds into which we can embed any surface, such as $M = \mathbb{RP}^3 \sharp \mathbb{RP}^3$. Indeed, by the classification of surfaces we know that any surface may be decomposed as a connected sum of copies of $\mathbb{RP}^2$ and tori. In fact, by the monoid structure of closed surfaces under connected sums we may take this sum to have at most 2 copies of the projective plane. Now, embed 2 disjoint copies of the projective plane into $M$ and arbitrarily many copies of the torus. Taking the connected sum of these we see that any closed surface is embeddable into $M$.

Can we do something similar in higher dimensions?

2 deleted 31 characters in body

Edited to fix the example, as per Zack's suggestion.

The Whitney embedding theorem states that any smooth $n$-manifold may be smoothly embedded into $\mathbb{R}^{2n}$. If we consider embeddings into more general $k$-dimensional manifolds, is it possible find a 'universal' manifold of dimension less than $2n$?

For example, a non-orientable 2-manifold cannot be embedded into $\mathbb{R}^3$, demonstrating the sharpness of the Whitney embedding theorem.

However, there are 3-manifolds into which we can embed any surface, such as $\mathbb{RP}^3$. M = \mathbb{RP}^3 \sharp \mathbb{RP}^3$. Indeed, by the classification of surfaces we know that any surface may be decomposed as a connected sum of copies of$\mathbb{RP}^2$and tori. In fact, by the monoid structure of closed surfaces under connected sums we may take this sum to have at most 2 copies of the projective plane. Now, embed 2 disjoint copies of the projective plane into$\mathbb{RP}^3$(say$[x:y:z] \mapsto [x:y:z:0]$and$[x:y:z] \mapsto [x:y:z:1]$in homogenous coordinates.) M$ and arbitrarily many copies of the torus. Taking the connected sum of these we see that any closed surface is embeddable into $\mathbb{RP}^3$.M$. Can we do something similar in higher dimensions? 1 # Is it possible to improve the Whitney embedding theorem? The Whitney embedding theorem states that any smooth$n$-manifold may be smoothly embedded into$\mathbb{R}^{2n}$. If we consider embeddings into more general$k$-dimensional manifolds, is it possible find a 'universal' manifold of dimension less than$2n$? For example, a non-orientable 2-manifold cannot be embedded into$\mathbb{R}^3$, demonstrating the sharpness of the Whitney embedding theorem. However, there are 3-manifolds into which we can embed any surface, such as$\mathbb{RP}^3$. Indeed, by the classification of surfaces we know that any surface may be decomposed as a connected sum of copies of$\mathbb{RP}^2$and tori. In fact, by the monoid structure of closed surfaces under connected sums we may take this sum to have at most 2 copies of the projective plane. Now, embed 2 disjoint copies of the projective plane into$\mathbb{RP}^3$(say$[x:y:z] \mapsto [x:y:z:0]$and$[x:y:z] \mapsto [x:y:z:1]$in homogenous coordinates.) and arbitrarily many copies of the torus. Taking the connected sum of these we see that any closed surface is embeddable into$\mathbb{RP}^3\$.

Can we do something similar in higher dimensions?