RE: Robots with knives have a lethal strike, study says
1.A robot may not injure a human being or, through inaction, allow a human being to come to harm.
2.A robot must obey any orders given to it by human beings, except where such orders would conflict with the First Law.
3.A robot must protect its own existence as long as such protection does not conflict with the First or Second Law.
But don't forget the Zeroth Law,
A robot may not harm humanity, or, by inaction, allow humanity to come to harm