# Projections on Neumann algebra

• Apr 21st 2011, 02:10 PM
karkusha
Projections on Neumann algebra
Hello, could you please help me to solve this problem:
let A is a von Neumann algebra, and ( $p_i$) - projections on A, $i\in I$. Then inf $p_i$, sup $p_i$ - projections on A.
Thank you!
• Apr 22nd 2011, 11:52 AM
Opalg
Quote:

Originally Posted by karkusha
Hello, could you please help me to solve this problem:
let A is a von Neumann algebra, and ( $p_i$) - projections on A, $i\in I$. Then inf $p_i$, sup $p_i$ - projections on A.

If $p$, $q$ are two projections in $A$, then $p\vee q$ is the range projection of $p+q$, and is therefore in $A.$ By induction, the sup of any finite family of projections in $A$ is also in $A.$

Given an infinite family, the directed net of sups of finite subfamilies (ordered by inclusion) is increasing, and bounded above by the identity. Therefore it converges strongly to a limit which is a projection and is therefore the sup of the whole family.

The result for infs follows by taking orthogonal complements.
• Apr 22nd 2011, 12:29 PM
karkusha
Thank you, Opalg. You explained that sup is in $A$, but why is sup projection on $A$?
• Apr 22nd 2011, 12:43 PM
Opalg
Quote:

Originally Posted by karkusha
Thank you, Opalg. You explained that sup is in $A$, but why is sup projection on $A$?

The sup of an increasing directed net of projections is equal to the sup projection (Kadison and Ringrose, Proposition 2.5.6).