Spitzer's formula

In probability theory, Spitzer's formula or Spitzer's identity gives the joint distribution of partial sums and maximal partial sums of a collection of random variables. The result was first published by Frank Spitzer in 1956.[1] The formula is regarded as "a stepping stone in the theory of sums of independent random variables".[2]

Statement of theorem

Let X₁, X₂, ... be independent and identically distributed random variables and define the partial sums S_n = X₁ + X₂ + ... + X_n. Define R_n = max(0,S₁,S₂,...,S_n). Then[3]

${\displaystyle \sum _{n=0}^{\infty }\phi _{n}(\alpha ,\beta )t^{n}=\exp \left[\sum _{n=1}^{\infty }{\frac {t^{n}}{n}}\left(u_{n}(\alpha )+v_{n}(\beta )-1\right)\right]}$

where

${\displaystyle {\begin{aligned}\phi _{n}(\alpha ,\beta )&=\operatorname {E} (\exp \left[i(\alpha R_{n}+\beta (R_{n}-S_{n})\right])\\u_{n}(\alpha )&=\operatorname {E} (\exp \left[i\alpha S_{n}^{+}\right])\\v_{n}(\beta )&=\operatorname {E} (\exp \left[i\beta S_{n}^{-}\right])\end{aligned}}}$

and S^± denotes (|S| ± S)/2.

Proof

Two proofs are known, due to Spitzer[1] and Wendel.[3]

References

1. Spitzer, F. (1956). "A combinatorial lemma and its application to probability theory". Transactions of the American Mathematical Society. 82 (2): 323–339. doi:10.1090/S0002-9947-1956-0079851-X.
2. Ebrahimi-Fard, K.; Guo, L.; Kreimer, D. (2004). "Spitzer's identity and the algebraic Birkhoff decomposition in pQFT". Journal of Physics A: Mathematical and General. 37 (45): 11037. arXiv:hep-th/0407082. Bibcode:2004JPhA...3711037E. doi:10.1088/0305-4470/37/45/020.
3. Wendel, James G. (1958). "Spitzer's formula: A short proof". Proceedings of the American Mathematical Society. 9 (6): 905–908. doi:10.1090/S0002-9939-1958-0103531-2. MR 0103531.