volterra series symbol

Hi, there
In volterra series literatures, the symbol ° can be found everywhere, such as in
V = B(w)°vs + B(w1,w2)°vs^2 + ....

what does this symbol mean?


TIA.

Added after 3 hours 48 minutes:

?

anyone has some ideas about this symbol?

it is very complicated in form
please referred the book
"VLSI for wireless communication" by B.Leung
there is some introduction

Thanks fanshuo
but I cannot get this book, any link to download that or any other reference papers?

you can find it in the ebook sector of this forum

cheers!

sometimes this symbol is used to represent the low-pass equivalent kernels related to baseband Volterra models

Reference
Principles of Digital Transmission: With Wireless Applications
S Benedetto, E Biglieri