top23 = function(n) {
    a = 2 ** floor(log(n) / log(2) - .5)
    while( a < n ) a = 2 * a
    m = a
    best = m
    while( a > 1 ) {
        a = a / 2
        m = m / 2
        while( m < n ) m = 3 * m
        if( m < best ) best = m
    }
    best
}