So, if we have an infinite dimensional Hilbert space, $H$ then the way you put a ring structure on $F(H)$ is by taking the isomorphism, $H\oplus H \to H$ we can define the sum of two Fredholm operators as $$ H \to H \oplus H \to H \oplus H \to H$$ where the middle map is the sum of the two operators.

What is the equivalent for the ring structure. I figured its something to do with: $$ H \to H \otimes H \oplus H \otimes H \to H \otimes H \oplus H \otimes H \to H$$ but I cannot get the signs on the image map to work out. Also, is there a good reference to all this? Especially on how this works out when we switch to spectra.

So, if we have an infinite dimensional Hilbert space $H$ then the way you put a ring structure on $F(H)$ is by taking the isomorphism $H\oplus H \to H$ we can define the sum of two Fredholm operators as $$ H \to H \oplus H \to H \oplus H \to H$$ where the middle map is the sum of the two operators.

What is the equivalent for the ring structure? I figured it's something to do with $$ H \to H \otimes H \oplus H \otimes H \to H \otimes H \oplus H \otimes H \to H$$ but I cannot get the signs on the image map to work out. Also, is there a good reference to all this? Especially on how this works out when we switch to spectra.