Why do these identical elementary integrations lead to differing results?
In the 1st version the parameters y,G are kept arbitrary before doing the
integration.
In the 2nd version the parameters are set before the integration.
The integrand is completely harmless, both with respect to the variable and
the parameters.
The results differ. Why?
x,y,G=sp.symbols('x y G')
c = lambda x: G/(x**2+G**2)
u=sp.integrate(c(x)*c(x-y),x)
d = (c(x)*c(x-y)).subs({y:sp.Integer(2),G:sp.Integer(1)})
v=sp.integrate(d,x)
print (u.subs({x:sp.Integer(1),y:sp.Integer(2),G:sp.Integer(1)})).evalf()
print (v.subs({x:sp.Integer(1)})).evalf()
-0.392699081698724 + 0.e-21*I
0
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