2x2 matrix [a, b ; c, d]
characteristic polynomial = -(b*c) + a*d + (-a - d)*x + x^2
discriminant = a^2 + 4*b*c - 2*a*d + d^2
eigenvalues:
(a + d + sqrt(discriminant))/2
(a + d - sqrt(discriminant))/2
3x3 matrices
NB: we use the variable j not i to avoid confusion with I as the square root of -1.
Collect[-1*CharacteristicPolynomial[ {{a,b,c},{d,e,f},{g,h,j}},x],x]//InputForm
c*e*g - c*d*h + b*d*j - a*e*j + (-(b*d) + a*e - b*g - c*g + a*h + a*j + e*j)*x + (-a - e - h - j)*x^2 + x^3
same thing but with array. beware that in Mathematica, array indicies start at 1. i don't understand why Mathematica puts the constant term between x^3 and x^2.
Collect[-1*CharacteristicPolynomial[ Array[a,{3,3}],x],x] //InputForm
x^3 + a[1, 3]*a[2, 2]*a[3, 1] - a[1, 2]*a[2, 3]*a[3, 1] - a[1, 3]*a[2, 1]*a[3, 2] + a[1, 1]*a[2, 3]*a[3, 2] + x^2*(-a[1, 1] - a[2, 2] - a[3, 3]) + a[1, 2]*a[2, 1]*a[3, 3] - a[1, 1]*a[2, 2]*a[3, 3] + x*(-(a[1, 2]*a[2, 1]) + a[1, 1]*a[2, 2] - a[1, 3]*a[3, 1] - a[2, 3]*a[3, 2] + a[1, 1]*a[3, 3] + a[2, 2]*a[3, 3])
to get eigenvalues, solve the characteristic polynomial above using the cubic equation. the discriminant allows peeking at the nature of roots before solving.
Discriminant[ CharacteristicPolynomial[ {{a,b,c},{d,e,f},{g,h,j}},x],x] //InputForm
a^2*b^2*d^2 + 4*b^3*d^3 - 2*a^3*b*d*e - 10*a*b^2*d^2*e + a^4*e^2 + 8*a^2*b*d*e^2 + b^2*d^2*e^2 - 2*a^3*e^3 - 2*a*b*d*e^3 + a^2*e^4 + 2*a^2*b*c*d*g + 12*b^2*c*d^2*g + 2*a^3*c*e*g - 2*a*b*c*d*e*g + 2*a^2*c*e^2*g + 20*b*c*d*e^2*g - 8*a*c*e^3*g + 4*c*e^4*g - 4*a^3*b*f*g - 18*a*b^2*d*f*g + 6*a^2*b*e*f*g - 18*b^2*d*e*f*g + 6*a*b*e^2*f*g - 4*b*e^3*f*g + a^2*c^2*g^2 + 12*b*c^2*d*g^2 + 8*a*c^2*e*g^2 - 8*c^2*e^2*g^2 - 18*a*b*c*f*g^2 + 36*b*c*e*f*g^2 - 27*b^2*f^2*g^2 + 4*c^3*g^3 - 4*a^3*c*d*h - 18*a*b*c*d^2*h + 6*a^2*c*d*e*h - 18*b*c*d^2*e*h + 6*a*c*d*e^2*h - 4*c*d*e^3*h + 4*a^4*f*h + 20*a^2*b*d*f*h + 12*b^2*d^2*f*h - 8*a^3*e*f*h - 2*a*b*d*e*f*h + 2*a^2*e^2*f*h + 2*b*d*e^2*f*h + 2*a*e^3*f*h - 18*a*c^2*d*g*h + 36*c^2*d*e*g*h + 20*a^2*c*f*g*h - 30*b*c*d*f*g*h - 38*a*c*e*f*g*h + 20*c*e^2*f*g*h + 36*a*b*f^2*g*h - 18*b*e*f^2*g*h + 12*c^2*f*g^2*h - 27*c^2*d^2*h^2 + 36*a*c*d*f*h^2 - 18*c*d*e*f*h^2 - 8*a^2*f^2*h^2 + 12*b*d*f^2*h^2 + 8*a*e*f^2*h^2 + e^2*f^2*h^2 + 12*c*f^2*g*h^2 + 4*f^3*h^3 + 2*a^3*b*d*j + 8*a*b^2*d^2*j - 2*a^4*e*j - 10*a^2*b*d*e*j + 8*b^2*d^2*e*j + 2*a^3*e^2*j - 10*a*b*d*e^2*j + 2*a^2*e^3*j + 2*b*d*e^3*j - 2*a*e^4*j - 2*a^3*c*g*j - 2*a*b*c*d*g*j - 10*a^2*c*e*g*j - 38*b*c*d*e*g*j + 20*a*c*e^2*g*j - 8*c*e^3*g*j + 6*a^2*b*f*g*j + 36*b^2*d*f*g*j - 24*a*b*e*f*g*j + 6*b*e^2*f*g*j - 10*a*c^2*g^2*j + 8*c^2*e*g^2*j - 18*b*c*f*g^2*j + 6*a^2*c*d*h*j + 36*b*c*d^2*h*j - 24*a*c*d*e*h*j + 6*c*d*e^2*h*j - 8*a^3*f*h*j - 38*a*b*d*f*h*j + 20*a^2*e*f*h*j - 2*b*d*e*f*h*j - 10*a*e^2*f*h*j - 2*e^3*f*h*j - 18*c^2*d*g*h*j - 2*a*c*f*g*h*j - 2*c*e*f*g*h*j - 18*b*f^2*g*h*j - 18*c*d*f*h^2*j + 8*a*f^2*h^2*j - 10*e*f^2*h^2*j + a^4*j^2 + 2*a^2*b*d*j^2 - 8*b^2*d^2*j^2 + 2*a^3*e*j^2 + 20*a*b*d*e*j^2 - 6*a^2*e^2*j^2 + 2*b*d*e^2*j^2 + 2*a*e^3*j^2 + e^4*j^2 + 8*a^2*c*g*j^2 + 20*b*c*d*g*j^2 - 10*a*c*e*g*j^2 + 2*c*e^2*g*j^2 + 6*a*b*f*g*j^2 + 6*b*e*f*g*j^2 + c^2*g^2*j^2 + 6*a*c*d*h*j^2 + 6*c*d*e*h*j^2 + 2*a^2*f*h*j^2 + 20*b*d*f*h*j^2 - 10*a*e*f*h*j^2 + 8*e^2*f*h*j^2 + 2*c*f*g*h*j^2 + f^2*h^2*j^2 - 2*a^3*j^3 - 8*a*b*d*j^3 + 2*a^2*e*j^3 - 8*b*d*e*j^3 + 2*a*e^2*j^3 - 2*e^3*j^3 - 2*a*c*g*j^3 + 2*c*e*g*j^3 - 4*b*f*g*j^3 - 4*c*d*h*j^3 + 2*a*f*h*j^3 - 2*e*f*h*j^3 + a^2*j^4 + 4*b*d*j^4 - 2*a*e*j^4 + e^2*j^4
Discriminant[ CharacteristicPolynomial[ Array[a,{3,3}],x],x] //InputForm
a[1, 1]^2*a[1, 2]^2*a[2, 1]^2 + 4*a[1, 2]^3*a[2, 1]^3 - 2*a[1, 1]^3*a[1, 2]*a[2, 1]*a[2, 2] - 10*a[1, 1]*a[1, 2]^2*a[2, 1]^2*a[2, 2] + a[1, 1]^4*a[2, 2]^2 + 8*a[1, 1]^2*a[1, 2]*a[2, 1]*a[2, 2]^2 + a[1, 2]^2*a[2, 1]^2*a[2, 2]^2 - 2*a[1, 1]^3*a[2, 2]^3 - 2*a[1, 1]*a[1, 2]*a[2, 1]*a[2, 2]^3 + a[1, 1]^2*a[2, 2]^4 + 2*a[1, 1]^2*a[1, 2]*a[1, 3]*a[2, 1]*a[3, 1] + 12*a[1, 2]^2*a[1, 3]*a[2, 1]^2*a[3, 1] + 2*a[1, 1]^3*a[1, 3]*a[2, 2]*a[3, 1] - 2*a[1, 1]*a[1, 2]*a[1, 3]*a[2, 1]*a[2, 2]*a[3, 1] + 2*a[1, 1]^2*a[1, 3]*a[2, 2]^2*a[3, 1] + 20*a[1, 2]*a[1, 3]*a[2, 1]*a[2, 2]^2*a[3, 1] - 8*a[1, 1]*a[1, 3]*a[2, 2]^3*a[3, 1] + 4*a[1, 3]*a[2, 2]^4*a[3, 1] - 4*a[1, 1]^3*a[1, 2]*a[2, 3]*a[3, 1] - 18*a[1, 1]*a[1, 2]^2*a[2, 1]*a[2, 3]*a[3, 1] + 6*a[1, 1]^2*a[1, 2]*a[2, 2]*a[2, 3]*a[3, 1] - 18*a[1, 2]^2*a[2, 1]*a[2, 2]*a[2, 3]*a[3, 1] + 6*a[1, 1]*a[1, 2]*a[2, 2]^2*a[2, 3]*a[3, 1] - 4*a[1, 2]*a[2, 2]^3*a[2, 3]*a[3, 1] + a[1, 1]^2*a[1, 3]^2*a[3, 1]^2 + 12*a[1, 2]*a[1, 3]^2*a[2, 1]*a[3, 1]^2 + 8*a[1, 1]*a[1, 3]^2*a[2, 2]*a[3, 1]^2 - 8*a[1, 3]^2*a[2, 2]^2*a[3, 1]^2 - 18*a[1, 1]*a[1, 2]*a[1, 3]*a[2, 3]*a[3, 1]^2 + 36*a[1, 2]*a[1, 3]*a[2, 2]*a[2, 3]*a[3, 1]^2 - 27*a[1, 2]^2*a[2, 3]^2*a[3, 1]^2 + 4*a[1, 3]^3*a[3, 1]^3 - 4*a[1, 1]^3*a[1, 3]*a[2, 1]*a[3, 2] - 18*a[1, 1]*a[1, 2]*a[1, 3]*a[2, 1]^2*a[3, 2] + 6*a[1, 1]^2*a[1, 3]*a[2, 1]*a[2, 2]*a[3, 2] - 18*a[1, 2]*a[1, 3]*a[2, 1]^2*a[2, 2]*a[3, 2] + 6*a[1, 1]*a[1, 3]*a[2, 1]*a[2, 2]^2*a[3, 2] - 4*a[1, 3]*a[2, 1]*a[2, 2]^3*a[3, 2] + 4*a[1, 1]^4*a[2, 3]*a[3, 2] + 20*a[1, 1]^2*a[1, 2]*a[2, 1]*a[2, 3]*a[3, 2] + 12*a[1, 2]^2*a[2, 1]^2*a[2, 3]*a[3, 2] - 8*a[1, 1]^3*a[2, 2]*a[2, 3]*a[3, 2] - 2*a[1, 1]*a[1, 2]*a[2, 1]*a[2, 2]*a[2, 3]*a[3, 2] + 2*a[1, 1]^2*a[2, 2]^2*a[2, 3]*a[3, 2] + 2*a[1, 2]*a[2, 1]*a[2, 2]^2*a[2, 3]*a[3, 2] + 2*a[1, 1]*a[2, 2]^3*a[2, 3]*a[3, 2] - 18*a[1, 1]*a[1, 3]^2*a[2, 1]*a[3, 1]*a[3, 2] + 36*a[1, 3]^2*a[2, 1]*a[2, 2]*a[3, 1]*a[3, 2] + 20*a[1, 1]^2*a[1, 3]*a[2, 3]*a[3, 1]*a[3, 2] - 30*a[1, 2]*a[1, 3]*a[2, 1]*a[2, 3]*a[3, 1]*a[3, 2] - 38*a[1, 1]*a[1, 3]*a[2, 2]*a[2, 3]*a[3, 1]*a[3, 2] + 20*a[1, 3]*a[2, 2]^2*a[2, 3]*a[3, 1]*a[3, 2] + 36*a[1, 1]*a[1, 2]*a[2, 3]^2*a[3, 1]*a[3, 2] - 18*a[1, 2]*a[2, 2]*a[2, 3]^2*a[3, 1]*a[3, 2] + 12*a[1, 3]^2*a[2, 3]*a[3, 1]^2*a[3, 2] - 27*a[1, 3]^2*a[2, 1]^2*a[3, 2]^2 + 36*a[1, 1]*a[1, 3]*a[2, 1]*a[2, 3]*a[3, 2]^2 - 18*a[1, 3]*a[2, 1]*a[2, 2]*a[2, 3]*a[3, 2]^2 - 8*a[1, 1]^2*a[2, 3]^2*a[3, 2]^2 + 12*a[1, 2]*a[2, 1]*a[2, 3]^2*a[3, 2]^2 + 8*a[1, 1]*a[2, 2]*a[2, 3]^2*a[3, 2]^2 + a[2, 2]^2*a[2, 3]^2*a[3, 2]^2 + 12*a[1, 3]*a[2, 3]^2*a[3, 1]*a[3, 2]^2 + 4*a[2, 3]^3*a[3, 2]^3 + 2*a[1, 1]^3*a[1, 2]*a[2, 1]*a[3, 3] + 8*a[1, 1]*a[1, 2]^2*a[2, 1]^2*a[3, 3] - 2*a[1, 1]^4*a[2, 2]*a[3, 3] - 10*a[1, 1]^2*a[1, 2]*a[2, 1]*a[2, 2]*a[3, 3] + 8*a[1, 2]^2*a[2, 1]^2*a[2, 2]*a[3, 3] + 2*a[1, 1]^3*a[2, 2]^2*a[3, 3] - 10*a[1, 1]*a[1, 2]*a[2, 1]*a[2, 2]^2*a[3, 3] + 2*a[1, 1]^2*a[2, 2]^3*a[3, 3] + 2*a[1, 2]*a[2, 1]*a[2, 2]^3*a[3, 3] - 2*a[1, 1]*a[2, 2]^4*a[3, 3] - 2*a[1, 1]^3*a[1, 3]*a[3, 1]*a[3, 3] - 2*a[1, 1]*a[1, 2]*a[1, 3]*a[2, 1]*a[3, 1]*a[3, 3] - 10*a[1, 1]^2*a[1, 3]*a[2, 2]*a[3, 1]*a[3, 3] - 38*a[1, 2]*a[1, 3]*a[2, 1]*a[2, 2]*a[3, 1]*a[3, 3] + 20*a[1, 1]*a[1, 3]*a[2, 2]^2*a[3, 1]*a[3, 3] - 8*a[1, 3]*a[2, 2]^3*a[3, 1]*a[3, 3] + 6*a[1, 1]^2*a[1, 2]*a[2, 3]*a[3, 1]*a[3, 3] + 36*a[1, 2]^2*a[2, 1]*a[2, 3]*a[3, 1]*a[3, 3] - 24*a[1, 1]*a[1, 2]*a[2, 2]*a[2, 3]*a[3, 1]*a[3, 3] + 6*a[1, 2]*a[2, 2]^2*a[2, 3]*a[3, 1]*a[3, 3] - 10*a[1, 1]*a[1, 3]^2*a[3, 1]^2*a[3, 3] + 8*a[1, 3]^2*a[2, 2]*a[3, 1]^2*a[3, 3] - 18*a[1, 2]*a[1, 3]*a[2, 3]*a[3, 1]^2*a[3, 3] + 6*a[1, 1]^2*a[1, 3]*a[2, 1]*a[3, 2]*a[3, 3] + 36*a[1, 2]*a[1, 3]*a[2, 1]^2*a[3, 2]*a[3, 3] - 24*a[1, 1]*a[1, 3]*a[2, 1]*a[2, 2]*a[3, 2]*a[3, 3] + 6*a[1, 3]*a[2, 1]*a[2, 2]^2*a[3, 2]*a[3, 3] - 8*a[1, 1]^3*a[2, 3]*a[3, 2]*a[3, 3] - 38*a[1, 1]*a[1, 2]*a[2, 1]*a[2, 3]*a[3, 2]*a[3, 3] + 20*a[1, 1]^2*a[2, 2]*a[2, 3]*a[3, 2]*a[3, 3] - 2*a[1, 2]*a[2, 1]*a[2, 2]*a[2, 3]*a[3, 2]*a[3, 3] - 10*a[1, 1]*a[2, 2]^2*a[2, 3]*a[3, 2]*a[3, 3] - 2*a[2, 2]^3*a[2, 3]*a[3, 2]*a[3, 3] - 18*a[1, 3]^2*a[2, 1]*a[3, 1]*a[3, 2]*a[3, 3] - 2*a[1, 1]*a[1, 3]*a[2, 3]*a[3, 1]*a[3, 2]*a[3, 3] - 2*a[1, 3]*a[2, 2]*a[2, 3]*a[3, 1]*a[3, 2]*a[3, 3] - 18*a[1, 2]*a[2, 3]^2*a[3, 1]*a[3, 2]*a[3, 3] - 18*a[1, 3]*a[2, 1]*a[2, 3]*a[3, 2]^2*a[3, 3] + 8*a[1, 1]*a[2, 3]^2*a[3, 2]^2*a[3, 3] - 10*a[2, 2]*a[2, 3]^2*a[3, 2]^2*a[3, 3] + a[1, 1]^4*a[3, 3]^2 + 2*a[1, 1]^2*a[1, 2]*a[2, 1]*a[3, 3]^2 - 8*a[1, 2]^2*a[2, 1]^2*a[3, 3]^2 + 2*a[1, 1]^3*a[2, 2]*a[3, 3]^2 + 20*a[1, 1]*a[1, 2]*a[2, 1]*a[2, 2]*a[3, 3]^2 - 6*a[1, 1]^2*a[2, 2]^2*a[3, 3]^2 + 2*a[1, 2]*a[2, 1]*a[2, 2]^2*a[3, 3]^2 + 2*a[1, 1]*a[2, 2]^3*a[3, 3]^2 + a[2, 2]^4*a[3, 3]^2 + 8*a[1, 1]^2*a[1, 3]*a[3, 1]*a[3, 3]^2 + 20*a[1, 2]*a[1, 3]*a[2, 1]*a[3, 1]*a[3, 3]^2 - 10*a[1, 1]*a[1, 3]*a[2, 2]*a[3, 1]*a[3, 3]^2 + 2*a[1, 3]*a[2, 2]^2*a[3, 1]*a[3, 3]^2 + 6*a[1, 1]*a[1, 2]*a[2, 3]*a[3, 1]*a[3, 3]^2 + 6*a[1, 2]*a[2, 2]*a[2, 3]*a[3, 1]*a[3, 3]^2 + a[1, 3]^2*a[3, 1]^2*a[3, 3]^2 + 6*a[1, 1]*a[1, 3]*a[2, 1]*a[3, 2]*a[3, 3]^2 + 6*a[1, 3]*a[2, 1]*a[2, 2]*a[3, 2]*a[3, 3]^2 + 2*a[1, 1]^2*a[2, 3]*a[3, 2]*a[3, 3]^2 + 20*a[1, 2]*a[2, 1]*a[2, 3]*a[3, 2]*a[3, 3]^2 - 10*a[1, 1]*a[2, 2]*a[2, 3]*a[3, 2]*a[3, 3]^2 + 8*a[2, 2]^2*a[2, 3]*a[3, 2]*a[3, 3]^2 + 2*a[1, 3]*a[2, 3]*a[3, 1]*a[3, 2]*a[3, 3]^2 + a[2, 3]^2*a[3, 2]^2*a[3, 3]^2 - 2*a[1, 1]^3*a[3, 3]^3 - 8*a[1, 1]*a[1, 2]*a[2, 1]*a[3, 3]^3 + 2*a[1, 1]^2*a[2, 2]*a[3, 3]^3 - 8*a[1, 2]*a[2, 1]*a[2, 2]*a[3, 3]^3 + 2*a[1, 1]*a[2, 2]^2*a[3, 3]^3 - 2*a[2, 2]^3*a[3, 3]^3 - 2*a[1, 1]*a[1, 3]*a[3, 1]*a[3, 3]^3 + 2*a[1, 3]*a[2, 2]*a[3, 1]*a[3, 3]^3 - 4*a[1, 2]*a[2, 3]*a[3, 1]*a[3, 3]^3 - 4*a[1, 3]*a[2, 1]*a[3, 2]*a[3, 3]^3 + 2*a[1, 1]*a[2, 3]*a[3, 2]*a[3, 3]^3 - 2*a[2, 2]*a[2, 3]*a[3, 2]*a[3, 3]^3 + a[1, 1]^2*a[3, 3]^4 + 4*a[1, 2]*a[2, 1]*a[3, 3]^4 - 2*a[1, 1]*a[2, 2]*a[3, 3]^4 + a[2, 2]^2*a[3, 3]^4
here is how Mathematica solves a monic cubic equation. better would be to avoid repeated subexpressions. this is likely not the Right Way for other reasons as well. the imaginary unit I that appears is casus irreducibilis.
Discriminant[ x^3+b*x^2+c*x+d,x] //InputForm
b^2*c^2 - 4*c^3 - 4*b^3*d + 18*b*c*d - 27*d^2
Reduce[ x^3+b*x^2+c*x+d==0,x,Cubics->True] //InputForm
x == -b/3 - (2^(1/3)*(-b^2 + 3*c))/ (3*(-2*b^3 + 9*b*c - 27*d + 3*Sqrt[3]*Sqrt[-(b^2*c^2) + 4*c^3 + 4*b^3*d - 18*b*c*d + 27*d^2])^(1/3)) + (-2*b^3 + 9*b*c - 27*d + 3*Sqrt[3]*Sqrt[-(b^2*c^2) + 4*c^3 + 4*b^3*d - 18*b*c*d + 27*d^2])^(1/3)/(3*2^(1/3)) || x == -b/3 + ((1 + I*Sqrt[3])*(-b^2 + 3*c))/ (3*2^(2/3)*(-2*b^3 + 9*b*c - 27*d + 3*Sqrt[3]*Sqrt[-(b^2*c^2) + 4*c^3 + 4*b^3*d - 18*b*c*d + 27*d^2])^(1/3)) - ((1 - I*Sqrt[3])*(-2*b^3 + 9*b*c - 27*d + 3*Sqrt[3]* Sqrt[-(b^2*c^2) + 4*c^3 + 4*b^3*d - 18*b*c*d + 27*d^2])^(1/3))/(6*2^(1/3)) || x == -b/3 + ((1 - I*Sqrt[3])*(-b^2 + 3*c))/ (3*2^(2/3)*(-2*b^3 + 9*b*c - 27*d + 3*Sqrt[3]*Sqrt[-(b^2*c^2) + 4*c^3 + 4*b^3*d - 18*b*c*d + 27*d^2])^(1/3)) - ((1 + I*Sqrt[3])*(-2*b^3 + 9*b*c - 27*d + 3*Sqrt[3]* Sqrt[-(b^2*c^2) + 4*c^3 + 4*b^3*d - 18*b*c*d + 27*d^2])^(1/3))/(6*2^(1/3))
The Roots function also works.
Can Mathematica be coaxed to emit the solution in terms of the discriminant?
4x4 matrices
Collect[CharacteristicPolynomial[ Array[a,{4,4}],x],x,Simplify] //InputForm
x^4 + a[1, 2]*a[2, 4]*a[3, 3]*a[4, 1] - a[1, 2]*a[2, 3]*a[3, 4]*a[4, 1] - a[1, 1]*a[2, 4]*a[3, 3]*a[4, 2] + a[1, 1]*a[2, 3]*a[3, 4]*a[4, 2] - a[1, 2]*a[2, 4]*a[3, 1]*a[4, 3] + a[1, 1]*a[2, 4]*a[3, 2]*a[4, 3] + a[1, 2]*a[2, 1]*a[3, 4]*a[4, 3] - a[1, 1]*a[2, 2]*a[3, 4]*a[4, 3] + a[1, 4]*(a[2, 3]*(a[3, 2]*a[4, 1] - a[3, 1]*a[4, 2]) + a[2, 2]*(-(a[3, 3]*a[4, 1]) + a[3, 1]*a[4, 3]) + a[2, 1]*(a[3, 3]*a[4, 2] - a[3, 2]*a[4, 3])) + x^3*(-a[1, 1] - a[2, 2] - a[3, 3] - a[4, 4]) + a[1, 2]*a[2, 3]*a[3, 1]*a[4, 4] - a[1, 1]*a[2, 3]*a[3, 2]*a[4, 4] - a[1, 2]*a[2, 1]*a[3, 3]*a[4, 4] + a[1, 1]*a[2, 2]*a[3, 3]*a[4, 4] + x^2*(-(a[1, 2]*a[2, 1]) - a[1, 3]*a[3, 1] - a[2, 3]*a[3, 2] + a[2, 2]*a[3, 3] - a[1, 4]*a[4, 1] - a[2, 4]*a[4, 2] - a[3, 4]*a[4, 3] + a[2, 2]*a[4, 4] + a[3, 3]*a[4, 4] + a[1, 1]*(a[2, 2] + a[3, 3] + a[4, 4])) + a[1, 3]*(a[2, 4]*(-(a[3, 2]*a[4, 1]) + a[3, 1]*a[4, 2]) + a[2, 2]*(a[3, 4]*a[4, 1] - a[3, 1]*a[4, 4]) + a[2, 1]*(-(a[3, 4]*a[4, 2]) + a[3, 2]*a[4, 4])) + x*(a[1, 1]*a[2, 3]*a[3, 2] - a[1, 1]*a[2, 2]*a[3, 3] + a[1, 4]*a[2, 2]*a[4, 1] + a[1, 4]*a[3, 3]*a[4, 1] - a[1, 4]*a[2, 1]*a[4, 2] + a[1, 1]*a[2, 4]*a[4, 2] + a[2, 4]*a[3, 3]*a[4, 2] - a[2, 3]*a[3, 4]*a[4, 2] - a[1, 4]*a[3, 1]*a[4, 3] - a[2, 4]*a[3, 2]*a[4, 3] + a[1, 1]*a[3, 4]*a[4, 3] + a[2, 2]*a[3, 4]*a[4, 3] - a[1, 1]*a[2, 2]*a[4, 4] + a[2, 3]*a[3, 2]*a[4, 4] - a[1, 1]*a[3, 3]*a[4, 4] - a[2, 2]*a[3, 3]*a[4, 4] + a[1, 3]*(a[2, 2]*a[3, 1] - a[2, 1]*a[3, 2] - a[3, 4]*a[4, 1] + a[3, 1]*a[4, 4]) + a[1, 2]*(-(a[2, 3]*a[3, 1]) - a[2, 4]*a[4, 1] + a[2, 1]*(a[3, 3] + a[4, 4])))
CoefficientList[ CharacteristicPolynomial[ Array[a,{4,4}],x],x] //InputForm
{a[1, 4]*a[2, 3]*a[3, 2]*a[4, 1] - a[1, 3]*a[2, 4]*a[3, 2]*a[4, 1] - a[1, 4]*a[2, 2]*a[3, 3]*a[4, 1] + a[1, 2]*a[2, 4]*a[3, 3]*a[4, 1] + a[1, 3]*a[2, 2]*a[3, 4]*a[4, 1] - a[1, 2]*a[2, 3]*a[3, 4]*a[4, 1] - a[1, 4]*a[2, 3]*a[3, 1]*a[4, 2] + a[1, 3]*a[2, 4]*a[3, 1]*a[4, 2] + a[1, 4]*a[2, 1]*a[3, 3]*a[4, 2] - a[1, 1]*a[2, 4]*a[3, 3]*a[4, 2] - a[1, 3]*a[2, 1]*a[3, 4]*a[4, 2] + a[1, 1]*a[2, 3]*a[3, 4]*a[4, 2] + a[1, 4]*a[2, 2]*a[3, 1]*a[4, 3] - a[1, 2]*a[2, 4]*a[3, 1]*a[4, 3] - a[1, 4]*a[2, 1]*a[3, 2]*a[4, 3] + a[1, 1]*a[2, 4]*a[3, 2]*a[4, 3] + a[1, 2]*a[2, 1]*a[3, 4]*a[4, 3] - a[1, 1]*a[2, 2]*a[3, 4]*a[4, 3] - a[1, 3]*a[2, 2]*a[3, 1]*a[4, 4] + a[1, 2]*a[2, 3]*a[3, 1]*a[4, 4] + a[1, 3]*a[2, 1]*a[3, 2]*a[4, 4] - a[1, 1]*a[2, 3]*a[3, 2]*a[4, 4] - a[1, 2]*a[2, 1]*a[3, 3]*a[4, 4] + a[1, 1]*a[2, 2]*a[3, 3]*a[4, 4], a[1, 3]*a[2, 2]*a[3, 1] - a[1, 2]*a[2, 3]*a[3, 1] - a[1, 3]*a[2, 1]*a[3, 2] + a[1, 1]*a[2, 3]*a[3, 2] + a[1, 2]*a[2, 1]*a[3, 3] - a[1, 1]*a[2, 2]*a[3, 3] + a[1, 4]*a[2, 2]*a[4, 1] - a[1, 2]*a[2, 4]*a[4, 1] + a[1, 4]*a[3, 3]*a[4, 1] - a[1, 3]*a[3, 4]*a[4, 1] - a[1, 4]*a[2, 1]*a[4, 2] + a[1, 1]*a[2, 4]*a[4, 2] + a[2, 4]*a[3, 3]*a[4, 2] - a[2, 3]*a[3, 4]*a[4, 2] - a[1, 4]*a[3, 1]*a[4, 3] - a[2, 4]*a[3, 2]*a[4, 3] + a[1, 1]*a[3, 4]*a[4, 3] + a[2, 2]*a[3, 4]*a[4, 3] + a[1, 2]*a[2, 1]*a[4, 4] - a[1, 1]*a[2, 2]*a[4, 4] + a[1, 3]*a[3, 1]*a[4, 4] + a[2, 3]*a[3, 2]*a[4, 4] - a[1, 1]*a[3, 3]*a[4, 4] - a[2, 2]*a[3, 3]*a[4, 4], -(a[1, 2]*a[2, 1]) + a[1, 1]*a[2, 2] - a[1, 3]*a[3, 1] - a[2, 3]*a[3, 2] + a[1, 1]*a[3, 3] + a[2, 2]*a[3, 3] - a[1, 4]*a[4, 1] - a[2, 4]*a[4, 2] - a[3, 4]*a[4, 3] + a[1, 1]*a[4, 4] + a[2, 2]*a[4, 4] + a[3, 3]*a[4, 4], -a[1, 1] - a[2, 2] - a[3, 3] - a[4, 4], 1}
one can extract items out of a list with the Part function. however, the fact that Mathematica indices start from 1 not 0 is especially confusing for CoefficientList.
discriminant is too huge to include. if you need it, construct it from the coefficients above and the discriminant of a general monic quartic given below.
ByteCount[ Discriminant[ CharacteristicPolynomial[ Array[a,{4,4}],x],x]]
74857232
(ByteCount of discriminant of characteristic polynomial of 3x3 matrix, given above, was 82072.)
solution to the monic quartic equation:
Discriminant[ x^4+b*x^3+c*x^2+d*x+e,x] //InputForm
b^2*c^2*d^2 - 4*c^3*d^2 - 4*b^3*d^3 + 18*b*c*d^3 - 27*d^4 - 4*b^2*c^3*e + 16*c^4*e + 18*b^3*c*d*e - 80*b*c^2*d*e - 6*b^2*d^2*e + 144*c*d^2*e - 27*b^4*e^2 + 144*b^2*c*e^2 - 128*c^2*e^2 - 192*b*d*e^2 + 256*e^3
Reduce[ x^4+b*x^3+c*x^2+d*x+e==0,x,Quartics->True] //InputForm
x == -b/4 - Sqrt[b^2/4 - (2*c)/3 + (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3))]/2 - Sqrt[b^2/2 - (4*c)/3 - (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) - (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3)) - (-b^3 + 4*b*c - 8*d)/ (4*Sqrt[b^2/4 - (2*c)/3 + (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3))])]/2 || x == -b/4 - Sqrt[b^2/4 - (2*c)/3 + (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3))]/2 + Sqrt[b^2/2 - (4*c)/3 - (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) - (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3)) - (-b^3 + 4*b*c - 8*d)/ (4*Sqrt[b^2/4 - (2*c)/3 + (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3))])]/2 || x == -b/4 + Sqrt[b^2/4 - (2*c)/3 + (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3))]/2 - Sqrt[b^2/2 - (4*c)/3 - (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) - (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3)) + (-b^3 + 4*b*c - 8*d)/ (4*Sqrt[b^2/4 - (2*c)/3 + (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3))])]/2 || x == -b/4 + Sqrt[b^2/4 - (2*c)/3 + (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3))]/2 + Sqrt[b^2/2 - (4*c)/3 - (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) - (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3)) + (-b^3 + 4*b*c - 8*d)/ (4*Sqrt[b^2/4 - (2*c)/3 + (2^(1/3)*(c^2 - 3*b*d + 12*e))/ (3*(2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)) + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e + Sqrt[-4*(c^2 - 3*b*d + 12*e)^3 + (2*c^3 - 9*b*c*d + 27*d^2 + 27*b^2*e - 72*c*e)^2])^(1/3)/(3*2^(1/3))])]/2
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