Degree-parity conjecture for regular maps from products of spheres

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Let nn and mm be positive integers. A continuous map is considered from the product of spheres Sn×Sm\mathbb{S}^n\times\mathbb{S}^m to Sn+m\mathbb{S}^{n+m}, and (n+mn)\binom{n+m}{n} denotes the corresponding binomial coefficient. Degree-parity conjecture. The following conditions are equivalent:

  1. Every continuous map from Sn×Sm\mathbb{S}^n\times\mathbb{S}^m into Sn+m\mathbb{S}^{n+m} is homotopic to a regular one.
  2. The number (n+mn)\binom{n+m}{n} is odd.

This conjecture extends the equivalence established in the paper for n≤3n\leq 3 and predicts a complete parity criterion for algebraic approximation of maps from products of spheres. The paper provides new families where the criterion holds, but does not state a resolution in general.

References

Primary source

Juliusz Banecki, “Invariants of real affine varieties based on their complexifications”, arXiv:2605.22941 (2026).

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