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]

\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

{\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]

gollark: You're going to have to scrub CO2 from its blood too.

gollark: It probably needs a bit of an immune system hooked up, or really good sterilization.

gollark: Well, it needs glucose, at least.

gollark: If by "people" you mean "nematode worms", possibly.

gollark: I feel like you might be underestimating the pickiness of biology.

References

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.
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.
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.

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[spitzer-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.

[wendel-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.