# normalized lower incomplete beta integral ibeta(a,b, x)
save_encoding = GPVAL_ENCODING
set encoding utf8

set title "Incomplete beta integral          {Γ(a+b)/(Γ(a)Γ(b))} {/*2 ∫@_{/*0.5 0}^{/*.5 x}} {/:Italic t^{a-1}(1-t)^{b-1}dt}\n" offset 0,-1

set xrange [0:1]
set yrange [0:1]

set xlabel "x"
set ylabel "ibeta(a,b, x)"
set key outside

plot ibeta(9,1,x) lw 2 title "a=9 b=1", ibeta(7,2,x) lw 2 title "a=7 b=2",\
     ibeta(5,5,x) lw 2 title "a=5 b=5", ibeta(2,7,x) lw 2 title "a=2 b=7",\
     ibeta(1,9,x) lw 2 lt 6 title "a=1 b=9"


Click here for minimal script to generate this plot

#
# Show region of the domain which the old ibeta algorithm did not cover
#

set xrange [0 : 1.0 ]
set yrange [0 : 0.1 ]
set zrange [0 : 1.0 ]
set xlabel "a"
set ylabel "b"
set xtics format "%.1f" offset 0, -0.3
set ytics format "%.2f"
set xyplane 0
set key at screen .8,.7 reverse right

set sample 40
set isosample 40

set pm3d lighting spec2 0.5 border lw .5
unset colorbox

z = 0.95
splot ibeta(x,y,z) with pm3d fc "gray75" title sprintf("ibeta(a,b, x=%.2f)",z)


Click here for minimal script to generate this plot

set encoding save_encoding