The Math Forum

Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Math Forum » Discussions » Inactive » comp.soft-sys.math.mathematica

Topic: Pattern with powers
Replies: 7   Last Post: Aug 8, 2013 12:02 AM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
Alexei Boulbitch

Posts: 483
Registered: 2/28/08
Re: Pattern with powers
Posted: Jul 31, 2013 4:49 AM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

I must admit that I am an absolute beginner in patterns, as I cannot cope with a little problem with patterns consisting of powers of variables x and y.

Specifically, I would like to select from a list all terms of the form

c x^u y^v (numerical coefficient c times x to the power u times y to the power v)

where u and v are allowed to take the values 0 and 1.

How can I do this using Cases?

I have already accomplished the first non trivial step using _. (blank followed by a dot) in order to get first powers of the variables:

ls = List@@Expand[5 (x + y)^3]
{5*x^3, 15*x^2*y, 15*x*y^2, 5*y^3}

Example 1
a = 2; Cases[ls, (_.)*x^(u_.)*y^(v_.) /; u >= a && v < a]
but misses the term

Example 2: this would be the form I would like most
Cases[ls, (_.)*x^_?(#1 >= a & )*y^_?(#1 < a & )]
Here even I didn't get the dot behind the blank before the test, so it misses first powers.

Thanks in advance for any help.

Best regards,

Hi, Wolfgang,

Your explanation is not quite clear. Have a look:

Clear[a, u, v, b];

a = 2;
Cases[ls, (Times[_, x, Power[y, v_]] /;
v <= a) | (Times[_, Power[x, u_]] /; u >= a)]

{5 x^3, 15 x^2 y, 15 x y^2}

Is it, what you are after? Or this:

Clear[a, u, v, b];
a = 2;
Cases[ls, (Times[_, x, Power[y, v_]] /;
v < a) | (Times[_, Power[x, u_]] /; u >= a)]

{5 x^3, 15 x^2 y}


Have fun, Alexei

Alexei BOULBITCH, Dr., habil.
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


Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2017. All Rights Reserved.