> Or it's tan-1 of 0. As arctan is usually defined, arctan( 0) = 0. I chose arccos for a reason. Inverse functions of periodic functions generally achieve unique results by careful limits on their domains. tan( n* pi) = 0, but arctan( 0) != n* pi. Advice on math and science found on comp.os.vms is generally unreliable, I claim.