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Evalutation of the following cell, which includes the symbolic resultant, of two univariate polynomials in $x$, with parameters $a,b,c$ for the first polynomial (of degree 4) and parameters $d,e,f,g$ for the second polynomial (of degree 8), just keeps running (for hours).

Why is it so slow, and how can it be made to be faster?

Code:

Resultant[x^4 - 4*(a + b + c)*x^3 + 2*(3*(a + b + c)^2 - 4*(a*b + a*c + b*c))*x^2 - 4*((a + b + c)^3 - 4*(a + b + c)*(a*b + a*c + b*c) + 16*a*b*c)*x+ (4*(a*b + a*c + b*c) - (a + b + c)^2)^2, x^8 - 8*(d + e + f + g)*x^7 + 4*(7*(d + e + f + g)^2 - 4*(d*e + d*f + e*f + d*g + e*g + f*g))*x^6 - 8*(7*(d + e + f + g)^3 - 12*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g) + 16*(d*e*f + d*e*g + d*f*g + e*f*g))*x^5 + 2*(-1088*(d*e*f*g) + 35*(d + e + f + g)^4 - 120*(d + e + f + g)^2*(d*e + d*f + e*f + d*g + e*g + f*g) + 48*(d*e + d*f + e*f + d*g + e*g + f*g)^2 + 256*(d + e + f + g)*(d*e*f + d*e*g + d*f*g + e*f*g))*x^4 - 8*(-320*(d*e*f*g)*(d + e + f + g) + 7*(d + e + f + g)^5 - 
 40*(d + e + f + g)^3*(d*e + d*f + e*f + d*g + e*g + f*g) + 48*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g)^2 + 
 96*(d + e + f + g)^2*(d*e*f + d*e*g + d*f*g + e*f*g) - 128*(d*e + d*f + e*f + d*g + e*g + f*g)*(d*e*f + d*e*g + d*f*g + e*f*g))*x^3 + 4*(320*(d*e*f*g)*(d + e + f + g)^2 + 7*(d + e + f + g)^6 - 1792*(d*e*f*g)*(d*e + d*f + e*f + d*g + e*g + f*g) - 
 60*(d + e + f + g)^4*(d*e + d*f + e*f + d*g + e*g + f*g) + 144*(d + e + f + g)^2*(d*e + d*f + e*f + d*g + e*g + f*g)^2 - 
 64*(d*e + d*f + e*f + d*g + e*g + f*g)^3 + 128*(d + e + f + g)^3*(d*e*f + d*e*g + d*f*g + e*f*g) - 
 512*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g)*(d*e*f + d*e*g + d*f*g + e*f*g) + 1024*(d*e*f*g)^2)*x^2 - 8*(192*(d*e*f*g)*(d + e + f + g)^3 + (d + e + f + g)^8 - 768*(d*e*f*g)*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g) - 
 12*(d + e + f + g)^5*(d*e + d*f + e*f + d*g + e*g + f*g) + 48*(d + e + f + g)^3*(d*e + d*f + e*f + d*g + e*g + f*g)^2 - 
 64*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g)^3 + 1024*(d*e*f*g)*(d*e*f + d*e*g + d*f*g + e*f*g) + 
 16*(d + e + f + g)^4*(d*e*f + d*e*g + d*f*g + e*f*g) - 128*(d + e + f + g)^2*(d*e + d*f + e*f + d*g + e*g + f*g)*(d*e*f + d*e*g + d*f*g + e*f*g) + 
 256*(d*e + d*f + e*f + d*g + e*g + f*g)^2*(d*e*f + d*e*g + d*f*g + e*f*g))*x + (((d + e + f + g)^2 - 4*(d*e + d*f + e*f + d*g + e*g + f*g))^2 - 64*(d*e*f*g))^2, x]
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1 Answer 1

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Making substitutions to the parameters make it a little faster. It took 500s to complete.

poly1 = x^4 - 4*(a + b + c)*x^3 + 
   2*(3*(a + b + c)^2 - 4*(a*b + a*c + b*c))*x^2 - 
   4*((a + b + c)^3 - 4*(a + b + c)*(a*b + a*c + b*c) + 16*a*b*c)*
    x + (4*(a*b + a*c + b*c) - (a + b + c)^2)^2;
poly2 = x^8 - 8*(d + e + f + g)*x^7 + 
   4*(7*(d + e + f + g)^2 - 4*(d*e + d*f + e*f + d*g + e*g + f*g))*
    x^6 - 8*(7*(d + e + f + g)^3 - 
      12*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g) + 
      16*(d*e*f + d*e*g + d*f*g + e*f*g))*x^5 + 
   2*(-1088*(d*e*f*g) + 35*(d + e + f + g)^4 - 
      120*(d + e + f + g)^2*(d*e + d*f + e*f + d*g + e*g + f*g) + 
      48*(d*e + d*f + e*f + d*g + e*g + f*g)^2 + 
      256*(d + e + f + g)*(d*e*f + d*e*g + d*f*g + e*f*g))*x^4 - 
   8*(-320*(d*e*f*g)*(d + e + f + g) + 7*(d + e + f + g)^5 - 
      40*(d + e + f + g)^3*(d*e + d*f + e*f + d*g + e*g + f*g) + 
      48*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g)^2 + 
      96*(d + e + f + g)^2*(d*e*f + d*e*g + d*f*g + e*f*g) - 
      128*(d*e + d*f + e*f + d*g + e*g + f*g)*(d*e*f + d*e*g + d*f*g +
          e*f*g))*x^3 + 
   4*(320*(d*e*f*g)*(d + e + f + g)^2 + 7*(d + e + f + g)^6 - 
      1792*(d*e*f*g)*(d*e + d*f + e*f + d*g + e*g + f*g) - 
      60*(d + e + f + g)^4*(d*e + d*f + e*f + d*g + e*g + f*g) + 
      144*(d + e + f + g)^2*(d*e + d*f + e*f + d*g + e*g + f*g)^2 - 
      64*(d*e + d*f + e*f + d*g + e*g + f*g)^3 + 
      128*(d + e + f + g)^3*(d*e*f + d*e*g + d*f*g + e*f*g) - 
      512*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g)*(d*e*f +
          d*e*g + d*f*g + e*f*g) + 1024*(d*e*f*g)^2)*x^2 - 
   8*(192*(d*e*f*g)*(d + e + f + g)^3 + (d + e + f + g)^8 - 
      768*(d*e*f*g)*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + 
         f*g) - 12*(d + e + f + g)^5*(d*e + d*f + e*f + d*g + e*g + 
         f*g) + 48*(d + e + f + g)^3*(d*e + d*f + e*f + d*g + e*g + 
          f*g)^2 - 
      64*(d + e + f + g)*(d*e + d*f + e*f + d*g + e*g + f*g)^3 + 
      1024*(d*e*f*g)*(d*e*f + d*e*g + d*f*g + e*f*g) + 
      16*(d + e + f + g)^4*(d*e*f + d*e*g + d*f*g + e*f*g) - 
      128*(d + e + f + g)^2*(d*e + d*f + e*f + d*g + e*g + 
         f*g)*(d*e*f + d*e*g + d*f*g + e*f*g) + 
      256*(d*e + d*f + e*f + d*g + e*g + f*g)^2*(d*e*f + d*e*g + 
         d*f*g + e*f*g))*
    x + (((d + e + f + g)^2 - 
         4*(d*e + d*f + e*f + d*g + e*g + f*g))^2 - 64*(d*e*f*g))^2;
rules = {a + b + c -> AA, a*b + a*c + b*c -> BB, a b c -> CC, 
   d + e + f + g -> DD, d e + d f + e f + d g + e g + f g -> EE, 
   d e f + d e g + d f g + e f g -> FF, d e f g -> GG, 
   d^2 e^2 f^2 g^2 -> GG^2};

poly1 = poly1 //. rules;
poly2 = poly2 //. rules;

AbsoluteTiming[res = Resultant[poly1, poly2, x];]

(*{493.44, Null}*)

res = res //. Map[Reverse, rules, {1}];
```
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