You probably know the Pascal triangle, of the binomial coefficients.
I draw it here for later reference. Let B(i,j) denote the j-th element of the
i-th row; the index i runs from 0 on, and j goes from 0 to i.
For example B(3,2) = 3.
1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 ...
2 1 2 1 3 2 1 4 5 2 1 5 9 7 2 1 6 14 16 9 2 ...If A(i,j) denotes the element in position j on the i-th row, then A(i,j) = A(i-1,j-1) + A(i-1,j).
1 1 -1 2 -3 1 4 -8 5 -1 8 -20 18 -7 1 ...Prove the following: cos(n x) = Sumk=0,.. n/2 T(n-k,k) cosn-2k(x)
cos(3x) = T(3,0) cos3(x) + T(2,1) cos(x) = 4 cos3(x) -3 cos(x) cos(4x) = T(4,0) cos4(x) + T(3,1) cos2(x) + T(2,2) = 8 cos4(x) - 8 cos2(x) + 1The claim explains why i called the triangle after Chebyshev. The polynomial expansions of cos(nx) in terms of powers of cos(x) are called Chebyshev polynomials of first kind, cos(n x)=Tn( cos(x) ). These Chebyshev polynomials are defined by the recurrence relation T(n+1)(x) = 2 x Tn(x) - Tn-1(x), with T0(x)=1 and T1(x)=x.
What about the Chebyshev polynomials of second kind? Let's go back to Pascal triangle and define D(i,j) = (-)i+j 2j B(i,j).
1 -1 2 1 -4 4 -1 6 -12 8 1 -8 24 -32 16 ...Next consider the polynomials (the sum runs over the k such that n+k is even)
Un(x) = Sumk D((n+k)/2, k) xkFor example,
U1(x) = D(1,1) x1 = 2 x U2(x) = D(1,0) x0 + D(2,2) x2 = -1 + 4 x2. U3(x) = D(2,1) x1 + D(3,3) x3 = -4 x + 8 x3.Prove that sin((n+1)x) = sin(x) Un( cos(x) ).
By the way, Pascal triangle is related to Fibonacci numbers. Write the triangle left-aligned,
1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 ...The sum of the diagonals are the Fibonacci numbers: 1, 1, 2, 3, 5, ... If you do the same for the Chebyshev triangle you get the Lucas numbers: 2, 1, 3, 4, 7, ...
Marco Corvi - 2005