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Composite function

asked 2011-04-09 12:32:57 -0500

BobB gravatar image

updated 2011-04-09 12:35:42 -0500

If I define:

var('x,y,z,t')

g(t) = (t, t^2, t^3)

f(x,y,z) = (2x,y+x+z,yx)

Is there a way for me to define the composite f(g(t)) without going through:

f(g(t)[0],g(t)[1],g(t)[2])

or something equally ugly?

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answered 2011-04-09 13:02:15 -0500

benjaminfjones gravatar image

If you let g be a tuple of expressions involving t you can use the Python * operator to use the tuple entries as your inputs x, y, and z:

sage: g = (t, t^2, t^3)
sage: f = lambda x,y,z: (2*x, y+x+z, y*x)
sage: f(*g)
(2*t, t^3 + t^2 + t, t^3)

If you want to define both g and f as lambda functions you could do:

sage: g = lambda t: (t, t^2, t^3)
sage: f = lambda x,y,z: (2*x, y+x+z, y*x)
sage: f(*g(t))
(2*t, t^3 + t^2 + t, t^3)
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Comments

You don't need f to be a lambda function for this; using "f(x,y,z) = (2*x,y+x+z,y*x)" works just fine.

John Palmieri gravatar imageJohn Palmieri ( 2011-04-09 13:07:54 -0500 )edit

Thank you for the alternative. For the moment it looks like non-lambda functions work well. I'm not well versed in python and I'd like to make things look as much like mathematical notation as possible. Also, it seems using other properties (methods, functions?) like jacobian and derivative is more straight forward if lambda functions are avoided.

BobB gravatar imageBobB ( 2011-04-09 22:05:12 -0500 )edit
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answered 2011-04-09 13:00:05 -0500

Ben Reynwar gravatar image

h(t) = f(*g(t))

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Great! And very quick, thank you. I suppose I would have known that if I knew something about python? Or is * a sage operator of some sort?

BobB gravatar imageBobB ( 2011-04-09 13:08:29 -0500 )edit

It's a python thing -- see benjaminfjones' answer.

John Palmieri gravatar imageJohn Palmieri ( 2011-04-09 13:09:22 -0500 )edit

But it might be nice to have this work without that. See http://trac.sagemath.org/sage_trac/ticket/11180; if someone can come up with a better title for that ticket, be my guest.

kcrisman gravatar imagekcrisman ( 2011-04-11 17:07:50 -0500 )edit

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Asked: 2011-04-09 12:32:57 -0500

Seen: 817 times

Last updated: Apr 09 '11