The Equivalence of Zero Span and Zero Semispan
James Francis Davis · Proceedings of the American Mathematical Society · 1984
In this paper we introduce the idea of the symmetric span of a continuum, and show that continua with zero symmetric span are in class $W$. Continua with zero span have zero symmetric span, but the converse does not hold. We also show that if every subcontinuum of the continuum $M$ is in class $W$ then the span of $M$ and the semispan of $M$ agree. These results are then applied to show that the properties of having zero span and of having zero semispan are equivalent.