def _rlFindStart(rl,lz,pot,lower=False):
    """find a starting interval for rl"""
    rtry= 2.*rl
    while (2.*lower-1.)*_rlfunc(rtry,lz,pot) > 0.:
        if lower:
            rtry/= 2.
        else:
            rtry*= 2.
    return rtry
find a starting interval for rl