International
Tables for
Crystallography
Volume B
Reciprocal space
Edited by U. Shmueli

International Tables for Crystallography (2010). Vol. B, ch. 1.3, p. 56   | 1 | 2 |

## Section 1.3.3.2.3.2. N a power of an odd prime

G. Bricognea

aGlobal Phasing Ltd, Sheraton House, Suites 14–16, Castle Park, Cambridge CB3 0AX, England, and LURE, Bâtiment 209D, Université Paris-Sud, 91405 Orsay, France

#### 1.3.3.2.3.2. N a power of an odd prime

| top | pdf |

This idea was extended by Winograd (1976 , 1978 ) to the treatment of prime powers , using the cyclic structure of the multiplicative group of units . The latter consists of all those elements of which are not divisible by p, and thus has elements. It is cyclic, and there exist primitive roots g modulo such that The elements divisible by p, which are divisors of zero, have to be treated separately just as 0 had to be treated separately for .

When , then with . The results are p-decimated, hence can be obtained via the -point DFT of the -periodized data Y: with When , then we may write where contains the contributions from and those from . By a converse of the previous calculation, arises from p-decimated data Z, hence is the -periodization of the -point DFT of these data: with (the -periodicity follows implicity from the fact that the transform on the right-hand side is independent of ).

Finally, the contribution from all may be calculated by reindexing by the powers of a primitive root g modulo , i.e. by writing then carrying out the multiplication by the skew-circulant matrix core as a convolution.

Thus the DFT of size may be reduced to two DFTs of size (dealing, respectively, with p-decimated results and p-decimated data) and a convolution of size . The latter may be `diagonalized' into a multiplication by purely real or purely imaginary numbers (because ) by two DFTs, whose factoring in turn leads to DFTs of size and . This method, applied recursively, allows the complete decomposition of the DFT on points into arbitrarily small DFTs.

### References

Winograd, S. (1976). On computing the discrete Fourier transform. Proc. Natl Acad. Sci. USA, 73, 1005–1006.
Winograd, S. (1978). On computing the discrete Fourier transform. Math. Comput. 32, 175–199.