This mode of convergence is called convergence in distribution.
The exact form of convergence is not just a technical nicety — the normal-
ized sums do not converge uniformly to a normal distribution. This means
that the tails of the distribution converge more slowly than its center. Es-
timates for the speed of convergence are given by the Berry-Ess´een theorem
and Chernoff’s bound.
The central limit theorem is true under wider conditions. We will be
able to prove it for independent variables with bounded moments, and even
more general versions are available. For example, limited dependency can
be tolerated (we will give a number-theoretic example). Moreover, random
variables not having moments (i.e. E[X
n
] doesn’t converge for all n) are
sometimes well-behaved enough to induce convergence. Other problematical
random variable will converge, under a different normalization, to an α-stable
distribution (look it up!).
2 Normal Distribution and Meaning of CLT
The normal distribution satisfies a nice convolution identity:
X
1
∼ N(µ
1
, σ
2
1
), X
2
∼ N(µ
2
, σ
2
2
) =⇒ X
1
+ X
2
∼ N(µ
1
+ µ
2
, σ
2
1
+ σ
2
2
).
Moreover, we can scale a normally distributed variable:
X ∼ N(µ, σ
2
) =⇒ cX ∼ N(cµ, c
2
σ
2
).
Even more exciting, we can recover the normal distribution from these prop-
erties. The equation N(0, 1)+N(0, 1) =
√
2N(0, 1) in essence defines N(0, 1)
(up to scaling), from which the entire ensemble can be recovered.
These properties point at why we should expect the normalized sums in
the central limit theorem to converge to a normal variable. Indeed, suppose
the convergence is to a hypothetical distribution D. From the equations
X
1
+ ··· + X
n
√
n
−→ D
X
1
+ ··· + X
2n
√
2n
−→ D
we would expect D + D =
√
2D, so D must be normal. Therefore the real
content of the central limit theorem is that convergence does take place. The
2