Re: defining an unknown function

*To*: mathgroup at smc.vnet.net*Subject*: [mg129451] Re: defining an unknown function*From*: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>*Date*: Mon, 14 Jan 2013 23:30:45 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net

Hi all, I would like to start by defining H = {{\partial_{x,x} L,\partial_{x,y} L}, {\partial_{y,x} L, \partial_{y,y}L}}; where at this stage, the only think i know is that L is of the form L(x,y). This is a Hessian, and I would like to symbolically compute the e-vals and e-vecs. I wanted to know how I should tell mathematica to consider L as L(x,y) ? Thanks, Sid. Hi, Sid, Is this: H[f_]:=Det[{{D[f,{x,2}],D[D[f,x],y]},{D[D[f,y],x],D[f,{y,2}]}}]//Simplify; H[Sin[x*y]] Cos[x y] (-Cos[x y]+2 x y Sin[x y]) the function that you mean? Have fun, Alexei Alexei BOULBITCH, Dr., habil. IEE S.A. ZAE Weiergewan, 11, rue Edmond Reuter, L-5326 Contern, LUXEMBOURG Office phone : +352-2454-2566 Office fax: +352-2454-3566 mobile phone: +49 151 52 40 66 44 e-mail: alexei.boulbitch at iee.lu<mailto:alexei.boulbitch at iee.lu>