A little long recursion

Algebra Level 4

If a a , b b and c c are complex numbers that satisfy: a 2 + b 2 + c 2 = 3 a^2+b^2+c^2=-3 a 3 + b 3 + c 3 = 46 a^3+b^3+c^3=-46 a 4 + b 4 + c 4 = 123 a^4+b^4+c^4=-123 And it's known that the value of a + b + c a+b+c is an integer.

Find ( a 10 + b 10 + c 10 ) m o d 1000 (a^{10}+b^{10}+c^{10}) \mod 1000 .


The answer is 162.

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2 solutions

Julian Poon
Dec 8, 2014

Im pretty surprised that after 2 hours of huge audience there isn't a single solution. I solved this quite a while ago so the solution does not have the small details

Let f ( n ) = a n + b n + c n f(n)={ a }^{ n }+{ b }^{ n }+{ c }^{ n } and a a , b b and c c be the roots of x 3 + m x 2 + n x + p { x }^{ 3 }+m{ x }^{ 2 }+nx+p

Using newton sums,

f ( 1 ) = m f ( 2 ) = m 2 2 n = 3 f ( 3 ) = m 3 + 3 m n 3 p = 46 f ( 4 ) = m 4 4 m 2 n + 4 m p + 2 n 2 = 123 f(1)=-m\\ f(2)={ m }^{ 2 }-2n=-3\\ f(3)=-{ m }^{ 3 }+3mn-3p=-46\\ f(4)={ m }^{ 4 }-4{ m }^{ 2 }n+4mp+2{ n }^{ 2 }=-123

It is pretty simple, now, just solve for m m and the answer would be m -m . Yeah... simple.

Using numerous substitutions, we get a Polynomial of degree 4

m 4 6 + 3 m 2 + 184 m 3 + 127.5 = 0 \frac { { m }^{ 4 } }{ 6 } +3{ m }^{ 2 }+\frac { 184m }{ 3 } +127.5=0

So, solving this tediously, (I used Vieta's formula), we get 2 2 real and 2 2 complex solutions:

m = 5 m=-5

m = 2.48411... m=-2.48411...

m = 3.74206... 6.89843... i m=3.74206...-6.89843...i

m = 3.74206... + 6.89843... i m=3.74206...+6.89843... i

Since m -m has to be an integer (stated), f ( 1 ) = 5 f(1)=5

Now, using newton sums again,

f ( 1 ) = S 1 = 5 f ( 2 ) = S 1 2 2 S 2 = 3 f ( 3 ) = S 1 f ( 2 ) S 2 f ( 1 ) + 3 S 3 = 46 f ( n ) = f ( 1 ) f ( n 1 ) S 2 f ( n 2 ) + S 3 f ( n 3 ) f(1)={ S }_{ 1 }=5\\ f(2)={ S }_{ 1 }^{ 2 }-2{ S }_{ 2 }=-3\\ f(3)={ S }_{ 1 }f(2)-{ S }_{ 2 }f(1)+{ 3S }_{ 3 }=-46\\ f(n)=f(1)f(n-1)-{ S }_{ 2 }f(n-2)+{ S }_{ 3 }f(n-3)

We can again tediously calculate that f ( 10 ) = 104838 f(10)=-104838

Therefore, the answer is 162 \boxed{162}

Thanks Alan Enrique Ontiveros Salazar! This question's rating has dropped by 58% since it's peak.

EDIT: Rating dropped by 73%. How many people solved this legidly?

Julian Poon - 6 years, 6 months ago

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That's unfair, the rating of my problem dropped from 220 to 65 now.

Alan Enrique Ontiveros Salazar - 6 years, 6 months ago
Cody Johnson
Dec 8, 2014

Read Julian Poon's solution on how to get that a , b , c a,b,c are roots of

0 = m 4 + 18 m 2 + 368 m + 765 = ( m + 5 ) ( m 3 5 m 2 + 43 m + 153 ) 0=m^4+18m^2+368m+765=(m+5)(m^3-5m^2+43m+153)

It's easy to see that 0 = m 3 5 m 2 + 43 m + 153 0=m^3-5m^2+43m+153 has no integer roots. However, it has at least one real root x x and two complex roots y y and z z . Since x + y + z = 5 x+y+z=5 from Vieta's formulas, and none of a , b , c a,b,c are integers, none of x + y x+y , y + z y+z , or z + x z+x are integers. If we add the other root 5 -5 to any of these, they still won't be integers. Therefore, the only possible combination of these roots such that they sum to an integer is the roots of 0 = m 3 5 m 2 + 43 m + 153 0=m^3-5m^2+43m+153 . This means that a , b , c a,b,c are the roots of 0 = m 3 5 m 2 + 43 m + 153 0=m^3-5m^2+43m+153 .

Now apply Newton's sums 10 10 times on the roots of this cubic equation to get the desired answer. There is no need to actually find the roots explicitly.

Did you mean that all the values of ( a + b + c ) -(a+b+c) are roots of m 4 + 18 m 2 + 368 m + 765 m^4+18m^2+368m+765 ? Because a a , b b and c c , for m = 5 m=-5 are roots of x 3 5 x 2 + 14 x 13 x^3-5x^2+14x-13 .

Alan Enrique Ontiveros Salazar - 6 years, 6 months ago

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