高校数学の質問スレ（医者・東大卒専用） Part438

# 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))