Register to reply 
2 questions on coholomogy groups 
Share this thread: 
#1
Sep811, 08:33 AM

P: 97

Question 1: $$0 \to A\mathop \to \limits^f B\mathop \to \limits^g C \to 0$$ is an exact short sequence,in order to prove $$\cdots \to H^q (A)\mathop \to \limits^{f^* } H^q (B)\mathop \to \limits^{g^* } H^q (C)\mathop \to \limits^{d^* } H^{q + 1} (A) \to \cdots$$ is an exact long cohomology sequence,we need to prove$${\mathop{\rm Im}\nolimits} f^* = \ker g^* $$,I can prove $$
{\mathop{\rm Im}\nolimits} f^* \subset \ker g^* $$ because $${\mathop{\rm Im}\nolimits} f = \ker g$$,but how to prove $ {\mathop{\rm Im}\nolimits} f^* \supset \ker g^* $? \\ \\ \\ Question 2: What makes the cohomology groups different by digging 2 points of $R^2$ from that of $R^2$ ?How does the closed and exact differential forms change? Thank you! the visualized questions are also in the attach files 


#2
Sep811, 01:51 PM

Sci Advisor
HW Helper
PF Gold
P: 4,771

A1: Simple diagram chasing. Take [b] in H^{q}(B), b in Ker d such that g*[b]=[g(b)]=0. I.e. g(b)=dc for some c in C^{q1}(C). To show: there exists a in C^{q}(A) and b' in C^{q1}(B) such that bdb'=f(a).
Choose b' in g^{1}(c) (which exists since g is surjective). Note that since g is a chain map, g(bdb')=g(b)dg(b')=dcdc=0. So, since Ker(g)=Im(f), there exists a in C^{q}(A) such that f(a)=bdb'. A2: This question is kinda vague, but notice that by digging points in R^2, new forms appear. Here is an example that illustrated the nature of the phenomenon: Consider on R²0 the 1form (ydx+xdy)/(x²+y²) [<ill defined on all of R²]. This form is closed but it is not exact. This is because it is equal to [itex]d\theta[/itex] everywhere where [itex]\theta[/itex] (the polar angle) is defined (i.e. R²  {a halfray}). Were (ydx+xdy)/(x²+y²) exact, this would imply that there exists a differentiable extension of the polar angle function [itex]\theta(x,y)[/itex] to all of R²0. But clearly, there exists no such even continuous extension. 


#3
Sep911, 09:02 AM

P: 97

Thank you very much! quasar987 :)



Register to reply 
Related Discussions  
3 questions about matrix lie groups  Linear & Abstract Algebra  3  
Some questions on vector fields on Lie groups  Calculus & Beyond Homework  0  
Questions involving simple groups  Calculus & Beyond Homework  9  
Questions aboug Special Groups SO(n) and SU(n)  Set Theory, Logic, Probability, Statistics  6 