Re: Coaxing N[] to work

*To*: mathgroup at smc.vnet.net*Subject*: [mg73379] Re: Coaxing N[] to work*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Thu, 15 Feb 2007 04:53:30 -0500 (EST)*Organization*: Uni Leipzig*References*: <equo6r$in0$1@smc.vnet.net>*Reply-to*: kuska at informatik.uni-leipzig.de

Hi, where should the 30 digits come from. You give x = {2.0, 3.0, 5.0}; that have 16 digits like a machine precision number The error propagation will not *increase* the precision and Mathematica has not enough imagination to add the missing digits. But x = SetPrecision[{2.0, 3.0, 5.0}, 30]; A = SetPrecision[{{6.0, 2.0, 1.0}, {2.0, 3.0, 1.0}, {1.0, 1.0, 1.0}}, 30]; For[k = 0, k < 15, ++k, lambda = x.A.x/(x.x); y = LinearSolve[A, x]; x = y/Norm[y, Infinity];] N[lambda, 30] may work. Regards Jens p at dirac.org wrote: > Sometimes N[,] doesn't appear to work. Like here: > > > x = {2.0, 3.0, 5.0}; > A = { {6.0, 2.0, 1.0}, {2.0, 3.0, 1.0}, {1.0, 1.0, 1.0} }; > For[ k=0, k<15, ++k, > lambda = x.A.x/(x.x); > y = LinearSolve[A,x]; > x = y / Norm[y,Infinity]; > ] > N[lambda, 30] > > > The output is: > > Out[5]= 0.578933 > > I was expecting 30 digits. Why did N[] ignore my request for 30 digits? >