I'm not sure it's possible not to have a default value! See e.g. this one - you just do n=5 or x_0=3 or whatever. http://sagecell.sagemath.org/?z=eJxzyMwrSS1KTC7h5UpJTVOI18izNdW04uVSAIKCIqCkQl6cEQDU7gqt&lang=sage If that's not what you meant, then you'll probably have to show us a sample to be able to help further.