correct f vector

f needs to be multiplied by dt/2

Author: waltherg

2013-12-03-Alternating_Direction_Implicit_Method.ipynb

@@ -673,7 +673,7 @@
      "cell_type": "code",
      "collapsed": false,
      "input": [
-      "f_vec = lambda U, V: numpy.multiply(dt, numpy.subtract(numpy.multiply(V, \n",
+      "f_vec = lambda U, V: numpy.multiply(float(dt)/float(2.), numpy.subtract(numpy.multiply(V, \n",
       "                     numpy.add(k0, numpy.divide(numpy.multiply(U,U), numpy.add(1., numpy.multiply(U,U))))), U))\n",
       "\n",
       "k0 = 0.067"