factor with 2 variables



On 02/21/2013 09:15 PM, Richard Fateman wrote:
> On 2/21/2013 12:26 PM, Ranousse wrote:
>> I wonder if there's a command to convert
>>   2      2/3  4/3      4/3  2/3    2
>> b  + 3 a    b    + 3 a    b    + a
>> into
>> (b^(2/3)+a^(2/3))^3
>>
>> Thank you.
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> no, but try
>  ratsubst(z,a^(3/2),%)
> ratsubst(w,b^(3/2),%)
> factor(%)
>
Hi,
Richard meant ratsubst(z,a^(2/3),%), ratsubst(w,b^(2/3),%), but that 
will not give you what you want. Try the following:

(%i2) f: b^2+3*a^(2/3)*b^(4/3)+3*a^(4/3)*b^(2/3)+a^2$

(%i3) subst([a=z^3,b=w^3],f);

(%o3) z^6+3*w^2*z^4+3*w^4*z^2+w^6

(%i4) factor(%);

(%o4) (z^2+w^2)^3

(%i5) subst([z=a^(1/3),w=b^(1/3)],%);

(%o5) (b^(2/3)+a^(2/3))^3

Cheers,
Jaime