how to get FFT of the polynomial

this is my code below and I tried to repeat it for 3 times so that I have a frequency of repeating so I can have a FFT, but I do not know how to get its FastfourierTransform:

%%%%Initial and final values definition %% qk=[0 2 0 2 0 2]; % Initial position qk1=[2 0 2 0 2 0 ]; % Final position dotqk=[0 0 0 0 0 0]; % Initial velocity dotqk1=[0 0 0 0 0 0]; % Final velocity ddotqk= [0 0 0 0 0 0]; % Initial acceleration ddotqk1= [0 0 0 0 0 0]; % Final acceleration dddotqk= [0 0 0 0 0 0]; % Initial jerk dddotqk1= [0 0 0 0 0 0]; % Final jerk tk= [0 1 2 3 4 5 6 ]; % Initial time tk1=[1 2 3 4 5 6]; % Final time tstep=1000; a=2; b=3;

%% polynomial 3 x3=zeros(4,numel(qk)); v3=zeros(3,numel(qk)); a3=zeros(2,numel(qk)); j3=zeros(1,numel(qk)); position3=zeros(numel(qk),tstep); velocity3=zeros(numel(qk),tstep); acceleration3=zeros(numel(qk),tstep); jerk3=zeros(numel(qk),tstep);

for i=1:numel(qk) x3(:,i)=pol3interpol(tk(i),tk1(i),qk(i),qk1(i),dotqk(i),dotqk1(i)); t(i,:) = linspace(tk(i),tk(i+1),tstep); position3(i,:)=polyval(x3(:,i),t(i,:)); v3(:,i)=polyder(x3(:,i)); velocity3(i,:)=polyval(v3(:,i),t(i,:)); a3(:,i)=polyder(v3(:,i)); acceleration3(i,:)=polyval(a3(:,i),t(i,:)); j3(:,i)=polyder(a3(:,i)); jerk3(i,:)=polyval(j3(:,i),t(i,:)); end

I have writen this code for fft but the result is strange:

% FFT

Fs = 1000; % Sampling frequency Ts = 1/Fs; % Sampling Period T=tk(1):Ts:tk(4)-Ts; L = length(T); % Signal length n = 2^nextpow2(L); f = Fs*(0:(n-1))/n;

% fft of Position position3T=(position3)'; pos3onerow=reshape(position3T,[],1); FFTpos3=fft(pos3onerow,n); abspos3=abs(FFTpos3); subplot(a,b,5) plot(f,abspos3); xlabel('frequency [Hz]') ylabel('|X(f)|') title('FFT of Trajectory Trend') grid on