v = s*ballFold(r, f*boxFold(v)) + c 

The Mandelbox equation is: v = s * ballFold(r, f*boxFold(v)) + c
where:
— v: 3d point
— boxFold(v) means for each axis a:

if v[a] > 1
v[a] = 2 – v[a]
else if v[a] < -1
v[a] = -2 – v[a]

— ballFold(r, v) means for v's magnitude m:

if m < r
m = m/r^2
else if m < 1
m = 1/m^2

The standard Mandelbox uses this formula with s=2, r=0.5 and f=1.