Take the 2-minute tour ×
MathOverflow is a question and answer site for professional mathematicians. It's 100% free, no registration required.

What is a Mackey Obstruction?

share|improve this question
3  
Hello, you are more likely to get an answer if you expand your question a little. What do you already know? What has led you to wanting to know about Mackey Obstructions? –  Andrew Stacey Apr 1 '11 at 12:17
    
Google is your friend, perhaps? –  Yemon Choi Apr 1 '11 at 20:57
add comment

closed as not a real question by Andrew Stacey, Dan Petersen, Daniel Moskovich, Leonid Positselski, Todd Trimble Apr 1 '11 at 18:33

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

2 Answers

I think, it is the obstruction of extending a projective representation of a group to a linear one considered as an element of some H^2 cohomology group. This is quite standard and classical. You may find the definition in e.g. :

Curtis, Charles W.; Reiner, Irving Representation theory of finite groups and associative algebras. Reprint of the 1962 original. AMS Chelsea Publishing, Providence, RI, 2006.

Curtis, Charles W.; Reiner, Irving Methods of representation theory. Vol. I. With applications to finite groups and orders. Reprint of the 1981 original. Wiley Classics Library. A Wiley-Interscience Publication. John Wiley & Sons, Inc., New York, 1990. xxiv+819 pp. ISBN: 0-471-52367-4, 20-02

share|improve this answer
add comment

For a succinct answer, read a bit of Section 2 of this paper.

Also, there is a survey (the article by Judith Packer) of projective representations and the Mackey obstruction. "Projective representations and the Mackey obstruction - a survey", Contemporary Mathematics, v. 449 (2008), pp. 345-378.

(I posted this same reference as an answer to another question this morning, and am surprised that both of these questions weren't asked by the same person. You two should get together and talk...)

share|improve this answer
add comment

Not the answer you're looking for? Browse other questions tagged or ask your own question.