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arXiv · 2610.26613

The two-step spectral locus of the octonionic structure operator

Sergey Grigorian
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Abstract

We continue the study of the octonionic spectrum of the octonionic structure operator $h$ on $W=\mathbb O\otimes_{\mathbb R}V$, where $V$ is the seven-dimensional representation of $G_2$. The real spectrum and the octonionic one-step spectrum, given by the equation $h(w)=wλ$ for real or octonion $λ$, have been determined in earlier work. In this paper, we study the two-step equation $h(w)=(wλ_1)λ_2$. Due to non-associativity, this does not in general reduce to the one-step problem. An adapted quaternionic frame represents successive right multiplication by an operator with four real coefficients. The $SU(2)$ subgroup fixing the frame splits $W$ into invariant sectors of dimensions $16$, $12$ and $28$. We factor the determinant of the resulting 4-parameter operator pencil and characterize the pairs for which a nonzero solution exists. The singular locus of this pencil in $\mathbb R^4$ is the union of an explicit algebraic set and a round $3$-sphere. The $12$-dimensional sector carries the one-step problem at the reversed product $λ_2λ_1$. The $28$-dimensional sector contributes no additional real singular points; the fixed sector produces solutions even when neither product lies in the one-step right spectrum.

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Sergey Grigorian. 2026-10-19. The two-step spectral locus of the octonionic structure operator. https://arxiv.org/abs/2610.26613

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