vectors with mma

*To*: mathgroup at yoda.physics.unc.edu*Subject*: vectors with mma*From*: BENNINGA at wilma.wharton.upenn.edu*Date*: Sun, 25 Oct 92 11:37 EDT

October 25, 1992 Dear Mathematica support: This is a not-especially intelligent query about (I think) writing a vector-dependent program in Mma. I'm trying to write a small program to do portfolio problems in finance. I want to both manipulate numbers and do analytics. Here's part of the program: Clear[S,X,Y,x,n,ER,ERp,Varp,Var,V] n=4; X=Array[x,n]; (* portfolio proportions *) Y=Array[ER,n]; (* vector of expected asset returns *) S=Array[V,{n,n}]; (* variance/covariance matrix *) ERp=X.Y; (* expected portfolio return *) Varp=X.S.X; (* portfolio variance *) D[ERp,x[1]] D[Varp,x[1]] This program produces the correct output: I.e., the last two lines produce the derviative of ERp wrt x[1] and the derivative of Varp wrt x[1]. Now suppose I try to put numerical values in for X, Y, and S. For instance, the next cell of this program has the following: Y={6,7,8,9}; S = {{10,2,4,5},{2,20,4,1},{4,4,40,10},{5,1,10,60}}; X = {0.4919379941,0.2998194609,0.1116229845,0.0966195605}; ERp I would have thought that this cell should produce a numerical answer. However, it only produces the following: ER[1] x[1] + ER[2] x[2] + ER[3] x[3] + ER[4] x[4] Can anyone tell me what I'm doing wrong? Simon Benninga Finance Department Wharton School University of Pennsylvania BENNINGA at WHARTON.UPENN.EDU FAX: 215-898-6200 TEL: 215-898-9466 P.S. As you can see, I'm new to Mma. I'm using version 2.1 for MS-DOS Windows. If anyone out there is doing finance with Mma, I'd be interested in making contact.