program fzero_main

! Main program for fzero 
! Adapted from Kahaner, Moler & Nash, "Numerical Methods and Software".

! Declare variables:
implicit none
integer :: iflag
real ::a, b, ae, re, f
external f  ! Required since f is an argument of fzero.

write(*,*) 'Enter interval endpoints:'
read*, a, b

write (*,*) 'Enter absolute and relative error tolerances ae,re:'
read*, ae, re

write (*,*) 'Initial interval: ', a, b
write(*,*) 'Tolerances:       ', ae, re

call fzero (f, a, b, b, re, ae, iflag)

write(*,*)
write(*,*) 'Fzero results:'
if (iflag/=1) write (*,*) 'Error code = ', iflag
write(*,*) 'Estimate of zero = ', a
write(*,*) 'Function value =  ', f(a)

stop

end program 
!==================================================================
function f(x)

! Declare variables:
implicit none
real :: f, x

f = x * (x*x-2.0) - 5.0

return
end function f