An N-periodic complex discrete-time sequence [x0,…,xN−1] can be resampled to an M-periodic sequence [y0,…,yM−1] with M>N, using sinc interpolation:
ym=∞∑n=−∞sinc(NmM−n)xnmodN=∞∑k=−∞N−1∑n=0sinc(NmM−n−Nk)xn
where mod denotes the modulo operation and:
sinc(x)={1if x=0,sin(πx)πxotherwise.
Eq. 1 can be seen as resampling an N-periodic continuous-time signal from samples xn at times n+Nk to samples ym at times NmM.
For example, a 2-periodic complex discrete-time sequence [x0,x1] can be resampled to a 4-periodic sequence [y0,y1,y2,y3]:
Eq. 1, N=2,M=4
The two series in Eq. 3 converge conditionally, with for example these possible rearrangements of the first series that give conflicting results if x0≠x1:
∞∑k=0(f(−k)+f(k+1))=x0+x12,∞∑k=0(f(−k)+f(2k+1)+f(2k+2))=(x1−x0)ln(2)2π+x0+x12,∞∑k=0(f(−k)+f(3k+1)+f(3k+2)+f(3k+3))=(x1−x0)ln(3)2π+x0+x12,
with shorthand f(k)=sinc(2k+32)x0+sinc(2k+12)x1.
Under which condition does the series given by Eq. 1 converge absolutely?
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