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  One question about "D-branes and K-Theory"

+ 2 like - 0 dislike
1297 views

I am trying to understand the equation

on page 4 of http://arxiv.org/pdf/hep-th/9810188v2.pdf

Making my proper computation I am obtaining

S8ch(V)=16i=1S8(eλi+eλi)=S8216p2(V)6=S8327683p2(V)=integer

and then we obtain

S8p2(V)=3k

where k is an integer.

But the equation (2.2) on page 3 of  http://arxiv.org/pdf/hep-th/9810188v2.pdf   says

S8p2(V)=6k

My question is if my computation is correct and how conciliate my results with (2.2) and (2.3) of  http://arxiv.org/pdf/hep-th/9810188v2.pdf .

asked Sep 10, 2015 in Theoretical Physics by juancho (1,130 points) [ revision history ]

2 Answers

+ 4 like - 0 dislike

It is just not true that 

S8ch(V)=iS8(eλi+eλi)

The correct formula is 

S8ch(V)=iS8(eλi+eλi)

(the Chern character is the SUM of the exponentials of the Chern roots).

From there it is easy to check the formula of the paper:

iS8(eλi+eλi)=iS82λ4i4!

and conclude using (ixi)2=ix2i+2i<jxixj with xi=λ2i and using the fact that p1(V)=iλ2i=0.

answered Sep 10, 2015 by 40227 (5,140 points) [ revision history ]

Many thanks for your answer.  I was confused about the relevant representation of SO(32) that is used in the Witten´s paper.  I was considering the spinor representation of SO(32) but Witten is using the adjoint representation of SO(32).  All is clear now.  Thank you.

+ 2 like - 0 dislike

Other form to derive (2.3) is as follows.

ch(V)=Tr(eiV2π)=Tr(1)+Tr(iV2π)+12!Tr([iV2π]2)+

13!Tr([iV2π]3)+14!Tr([iV2π]4)+...

Taking the 8-form we have

ch(V)8form=14!Tr([iV2π]4)=1384Tr(V4)π4

Now given that

p1(V)=18Tr(V2)π2

p2(V)=11282Tr(V4)+(Tr(V2))2π4

and using that p1(V)=0 we obtain Tr(V2)=0 and then

p2(V)=11282Tr(V4)π4=164Tr(V4)π4

which is equivalent to

Tr(V4)=64p2(V)π4

Using this last result we obtain finally

ch(V)8form=16p2(V)

answered Sep 10, 2015 by juancho (1,130 points) [ revision history ]
edited Sep 10, 2015 by juancho

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