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Brownian bridge : ウィキペディア英語版
Brownian bridge

A Brownian bridge is a continuous-time stochastic process ''B''(''t'') whose probability distribution is the conditional probability distribution of a Wiener process ''W''(''t'') (a mathematical model of Brownian motion) given the condition that ''B''(1) = 0. More precisely:
: B_t := (W_t|W_1=0),\;t \in ()
The expected value of the bridge is zero, with variance ''t''(1 − ''t''), implying that the most uncertainty is in the middle of the bridge, with zero uncertainty at the nodes. The covariance of ''B''(''s'') and ''B''(''t'') is ''s''(1 − ''t'') if ''s'' < ''t''.
The increments in a Brownian bridge are not independent.
== Relation to other stochastic processes ==
If ''W''(''t'') is a standard Wiener process (i.e., for ''t'' ≥ 0, ''W''(''t'') is normally distributed with expected value 0 and variance ''t'', and the increments are stationary and independent), then
: B(t) = W(t) - t W(1)\,
is a Brownian bridge for ''t'' ∈ (). It is independent of ''W''(''1'')〔Aspects of Brownian motion, Springer, 2008, R. Mansuy, M. Yor page 2〕
Conversely, if ''B''(''t'') is a Brownian bridge and ''Z'' is a standard normal random variable independent of ''B'', then the process
: W(t) = B(t) + tZ\,
is a Wiener process for ''t'' ∈ (). More generally, a Wiener process ''W''(''t'') for ''t'' ∈ () can be decomposed into
: W(t) = B\left(\frac\right) + \frac\right).
Conversely, for ''t'' ∈ ()
: W(t) = (1+t) B\left(\frac\right).
The Brownian bridge may also be represented as a Fourier series with stochastic coefficients, as
: B_t = \sum_^\infty Z_k \frac
where Z_1, Z_2, \ldots are independent identically distributed standard normal random variables (see the Karhunen–Loève theorem).
A Brownian bridge is the result of Donsker's theorem in the area of empirical processes. It is also used in the Kolmogorov–Smirnov test in the area of statistical inference.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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