高校数学の質問スレ（医者・東大卒専用） Part438 (991ﾚｽ)
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ﾘﾛｰﾄﾞ規制です｡10分ほどで解除するので､他のﾌﾞﾗｳｻﾞへ避難してください｡

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847(1): 05/25(日)04:22 ID:P4nhnL8B(1/6) AAS
# dbeta(L,a,b) == dbbeta(U,a,b)
# Solve[L^(a-1)(1-L)^(b-1)==U^(a-1)(1-U)^(b-1), b]

L=1/7
U=1/5
credMass = 0.95
f = function(a) 1 + ((a - 1) * log(U / L)) / log((1 - L) / (1 - U))
g = function(a) pbeta(U,a,f(a)) - pbeta(L,a,f(a)) - credMass
(re=uniroot(g,c(1,1e5)))
a=re$root
b=f(a)
c(a,b)
curve(g(x),1,1.5*a,bty="l") ; abline(h=0,lty=3)
a/(a+b) # mean
(a-1)/(a-1+b-1) # mode

library(LearnBayes)
beta.select(list(x=1/7,p=0.025),list(x=1/5,p=0.975))

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849: 05/25(日)06:42 ID:P4nhnL8B(3/6) AAS
>>847
このプロトタイプをAIに与えて描画機能やコメントをつけてもらった。

beta.parameter <- function(lower, upper, credMass = 0.95, verbose = FALSE) {

# Helper function to convert decimal numbers to fraction strings using MASS::fractions
fractionStr <- function(x) {
as.character(MASS::fractions(x))
}

# Function to compute beta parameter (beta) based on alpha,
# derived from the condition on the shape of the distribution between lower and upper
f <- function(alpha) {
1 + ((alpha - 1) * log(upper / lower)) / log((1 - lower) / (1 - upper))
}

# Root-finding function: difference between desired credible mass
# and the Beta CDF probability between lower and upper with parameters (alpha, f(alpha))
g <- function(alpha) {
pbeta(upper, alpha, f(alpha)) - pbeta(lower, alpha, f(alpha)) - credMass
}

# Find the root of g(alpha) = 0 over the interval [1, 1e5]
# to find the alpha value that satisfies the credible mass condition
re <- uniroot(g, c(1, 1e5))
alpha <- re$root
beta <- f(alpha)

# Calculate the mean of the Beta distribution
mean <- alpha / (alpha + beta)

# Calculate the mode if defined (alpha > 1 and beta > 1),
# otherwise set mode to NA as it is undefined
mode <- if (alpha > 1 && beta > 1) {
(alpha - 1) / (alpha + beta - 2)
} else {
NA
}

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