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In mathematics, given a linear space X, a set \( A \subseteq X \) is radial at the point \( x_0 \in A \) if for every \( x \in X \) there exists a t_x > 0 such that for every \( t \in [0,t_x], x_0 + tx \in A \).[1] In set notation, A is radial at the point \( x_0 \in A \) if

\( \bigcup_{x \in X}\ \bigcap_{t_x > 0}\ \bigcup_{t \in [0,t_x]} \{x_0 + tx\} \subseteq A.

The set of all points at which \( A \subseteq X \) is radial is equal to the algebraic interior.[1][2] The points at which a set is radial are often referred to as internal points.[3][4]

A set \( A \subseteq X \) is absorbing if and only if it is radial at 0.[1] Some authors use the term radial as a synonym for absorbing, i. e. they call a set radial if it is radial at 0.[5]


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Nikolaĭ Kapitonovich Nikolʹskiĭ (1992). Functional analysis I: linear functional analysis. Springer. ISBN 978-3-540-50584-6.
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