EXTERNAL

SUB mnbrak (ax, bx, cx, fa, fb, fc, dum)

    DECLARE FUNCTION func   ! Supplied by the user

    LET gold = 1.618034
    LET glimit = 100
    LET tiny = 1e-20

    LET fa = func(ax)
    LET fb = func(bx)
    IF fb > fa then
       LET dum = ax
       LET ax = bx
       LET bx = dum
       LET dum = fb
       LET fb = fa
       LET fa = dum
    END IF
    LET cx = bx + gold * (bx - ax)
    LET fc = func(cx)
    DO while fb > fc
       LET r = (bx - ax) * (fb - fc)
       LET q = (bx - cx) * (fb - fa)
       LET dum = q - r
       IF abs(dum) < tiny then LET dum = tiny
       LET u = bx - ((bx - cx) * q - (bx - ax) * r) / (2 * dum)
       LET ulim = bx + glimit * (cx - bx)
       IF (bx - u) * (u - cx) > 0 then
          LET fu = func(u)
          IF fu < fc then
             LET ax = bx
             LET fa = fb
             LET bx = u
             LET fb = fu
             EXIT SUB
          ELSEIF fu > fb then
             LET cx = u
             LET fc = fu
             EXIT SUB
          END IF
          LET u = cx + gold * (cx - bx)
          LET fu = func(u)
       ELSEIF (cx - u) * (u - ulim) > 0 then
          LET fu = func(u)
          IF fu < fc then
             LET bx = cx
             LET cx = u
             LET u = cx + gold * (cx - bx)
             LET fb = fc
             LET fc = fu
             LET fu = func(u)
          END IF
       ELSEIF (u - ulim) * (ulim - cx) >= 0 then
          LET u = ulim
          LET fu = func(u)
       ELSE
          LET u = cx + gold * (cx - bx)
          LET fu = func(u)
       END IF
       LET ax = bx
       LET bx = cx
       LET cx = u
       LET fa = fb
       LET fb = fc
       LET fc = fu
    LOOP

END SUB