# Nonimmersions of RP^n implied by tmf, revisited

## TL;DR

This paper revisits and corrects previous results on nonimmersions of real projective spaces using tmf, providing new proofs, filling gaps, and fully determining related tmf cohomology groups.

## Contribution

It corrects and completes earlier nonimmersion results for RP^n using tmf, and determines specific tmf cohomology groups for product spaces.

## Key findings

- Confirmed nonimmersion results for RP^n with n as small as 48
- Filled a gap in the proof regarding axial maps and cohomology classes
- Determined tmf^{8*}(RP^
imes RP^
) and tmf^*(CP^
imes CP^
) in positive dimensions

## Abstract

In a 2002 paper, the authors and Bruner used the new spectrum tmf to obtain some new nonimmersions of real projective spaces. In this note, we complete/correct two oversights in that paper. The first is to note that in that paper a general nonimmersion result was stated which yielded new nonimmersions for RP^n with n as small as 48, and yet it was stated there that the first new result occurred when n=1536. Here we give a simple proof of those overlooked results. Secondly, we fill in a gap in the proof of the 2002 paper. There it was claimed that an axial map f must satisfy f^*(X)=X_1+X_2. We realized recently that this is not clear. However, here we show that it is true up multiplication by a unit in the appropriate ring, and so we retrieve all the nonimmersion results claimed in the original paper. Finally, we present a complete determination of tmf^{8*}(RP^\infty\times RP^\infty) and tmf^*(CP^\infty\times CP^\infty) in positive dimensions.