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)8−form=14!Tr([iV2π]4)=1384Tr(V4)π4
Now given that
p1(V)=−18Tr(V2)π2
p2(V)=1128−2Tr(V4)+(Tr(V2))2π4
and using that p1(V)=0 we obtain Tr(V2)=0 and then
p2(V)=1128−2Tr(V4)π4=−164Tr(V4)π4
which is equivalent to
Tr(V4)=−64p2(V)π4
Using this last result we obtain finally
ch(V)8−form=−16p2(V)