The Finite Subsets of Z 2 Having the Extension Property
Mihály Bakonyi, Geir Nævdal · Journal of the London Mathematical Society · 2000
The paper considers finite subsets Λ ⊂ Zd which possess the extension property, namely that every collection {ck}k∈Λ−Λ of complex numbers which is positive definite with respect to Λ is the restriction of the Fourier coefficients of some positive measure on Td. All finite subsets of Z2 which possess the extension property are described.