Coincidence conjecture for optimal concatenation indices of primitive stepsize schedules

Let h(n)h_{\circ}^{(n)} denote the primitive stepsize schedule generated by Definition, let \CallConPP\Call{ConPP} denote the concatenation operation, and let 1\mathbf 1 be the all-ones vector. Coincidence conjecture. For each nNn\in\mathbb{N}, it holds that

1Th(n)=1T\CallConPPh(n1)/2,h(n1)/2.\mathbf 1^T h_{\circ}^{(n)} = \mathbf 1^T\Call{ConPP}{h_{\circ}^{\lfloor (n-1)/2\rfloor}, h_{\circ}^{\lceil (n-1)/2\rceil}}.

Moreover, if n=p2l1n=p\cdot 2^l-1, where p>1p>1 is an odd integer and lNl\in\mathbb{N}, then

1Th(n)=1T\CallConPPh((p+i)2l11),h((pi)2l11),i{1,0,1}.\mathbf 1^T h_{\circ}^{(n)} = \mathbf 1^T\Call{ConPP}{h_{\circ}^{((p+i)\cdot 2^{l-1}-1)},h_{\circ}^{((p-i)\cdot 2^{l-1}-1)}},\qquad i\in\{-1,0,1\}.

The conjecture formalizes the observed non-uniqueness of optimal concatenation indices: the balanced split is optimal, with additional optimal splits in the stated dyadic cases. Its status is unresolved in the supplied text and is supported only by simple examples and numerical tests.

Sources & referencesView supporting material

Primary source

Zehao Zhang and Rujun Jiang, “Accelerated Gradient Descent by Concatenation of Stepsize Schedules”, arXiv:2410.12395 (2026).

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