# Exp(i infinity) = 0 ?

kakarukeys
:grumpy: :yuck:
Hello there,

please tell me whether $$e^{iS} = 0$$ when $$S = +\infty$$

S is the action of a particle $$= \int L dt$$
e.g. when the particle goes to infinity and comes back, kinetic energy blows up,
then S blows up.

Gold Member
kakarukeys said:
:grumpy: :yuck:
Hello there,

please tell me whether $$e^{iS} = 0$$ when $$S = +\infty$$
It does'nt:
$$e^{ix}=\cos{x}+i\sin{x}$$
$$\lim_{x\rightarrow \infty}e^{ix}=\lim_{x\rightarrow \infty}\cos{x}+i\lim_{x\rightarrow \infty}\sin{x}$$
But niether the real nor imaginary parts of the limit exist; as x approaches infinity they oscilate between -1 and 1.

kakarukeys
That's what my math teacher told me,
but my physics teacher told me entirely different thing.

Staff Emeritus
Gold Member
I would suspect both your math teacher and physics teacher are right!

The map x → e^(ix) is, indeed, discontinuous at +∞, and thus one cannot continuously extend this map to have a value at +∞. (continuous extension is the process that is generally used to justify statements like arctan +∞ = π / 2 or x/x = 1).

However...

As is often the case, there is probably some related concept that is practical to use here, and whether he knows it or not, it's what your physics professor meant.

kakarukeys
Do you mean, for computation purpose, we can define the limit to be zero
it can't be proven?

kakarukeys

this is no college level!
Feymann Path Integral!

inha
kakarukeys said:

this is no college level!
Feymann Path Integral!

College=university. Don't get freaked out.

kakarukeys
Feynmann Path Integral is at graduate level :rofl: