Lieber Herr Büttiker,
Sie haben das Evaluate in der Plot-Funktion vergessen! So läuft alles prima:
KK[s_, t_, t1_, t2_] :=
t^2 + t1^2 + t2^2 - 2(t t1 + t t2 + t1 t2) - t t1 t2 / ((s - 4)/4);
II[s_, t_, t1_, t2_] := Module[{ns1, ns2, aux, int1, int2, tbar, tt},
aux = Solve[KK[s, tt, t1, t2] == 0, tt];
ns1 = Min[tt /. aux];
ns2 = Max[tt /. aux];
aux = Simplify[KK[s, tbar, t1, t2]^(-1/2)/(tbar - t)];
int2 = Integrate[aux, {tbar, ns2, Infinity}];
int2
]
Plot[II[5, t, 4, 88]//Evaluate, {t, 5, 6}]
MfG
Stefan Welke