# A proof of Wright's conjecture

## TL;DR

This paper proves Wright's conjecture for the entire parameter range by analyzing the neighborhood of the Hopf bifurcation at  = /2, confirming the global attractor property of the origin for the delay differential equation.

## Contribution

The authors extend the proof of Wright's conjecture to all  /2, including the bifurcation point, using a detailed bifurcation analysis and ruling out subsequent bifurcations of periodic orbits.

## Key findings

- Confirmed the origin as the global attractor for all  /2.
- Showed the branch of periodic orbits has no further bifurcations near the bifurcation point.
- Established the global parametrization of the periodic orbit branch by  > /2.

## Abstract

Wright's conjecture states that the origin is the global attractor for the delay differential equation $y'(t) = - \alpha y(t-1) [ 1 + y(t) ] $ for all $\alpha \in (0,\tfrac{\pi}{2}]$. This has been proven to be true for a subset of parameter values $\alpha$. We extend the result to the full parameter range $\alpha \in (0,\tfrac{\pi}{2}]$, and thus prove Wright's conjecture to be true. Our approach relies on a careful investigation of the neighborhood of the Hopf bifurcation occurring at $\alpha =\tfrac{\pi}{2}$. This analysis fills the gap left by complementary work on Wright's conjecture, which covers parameter values further away from the bifurcation point. Furthermore, we show that the branch of (slowly oscillating) periodic orbits originating from this Hopf bifurcation does not have any subsequent bifurcations (and in particular no folds) for $\alpha\in(\tfrac{\pi}{2} , \tfrac{\pi}{2} + 6.830 \times 10^{-3}]$. When combined with other results, this proves that the branch of slowly oscillating solutions that originates from the Hopf bifurcation at $\alpha=\tfrac{\pi}{2}$ is globally parametrized by $\alpha > \tfrac{\pi}{2}$.