Quick easy question: what is the meaning of the symbol $(\space\space )$. I've seen it now in two papers, one of which is Milgram's Group Representations and the Adams Spectral Sequence, avaiable at here.

On the top of page 170, Milgram writes $\partial(\space\space) \to S^k\vee S^k\to S^k\to v$

I figure if I knew what was meant by mathematical colloquialism "$\partial(\space \space)$", interpereting this part of the paper would be easy enough, I'm just not sure wht it means.

Thanks