  The central concept in the harmonic analysis of a compact group is the
completeness of Peter-Weyl orthonormal basis as constructed from the matrix
coefficients of a maximal set of irreducible unitary representations of the
group, leading ultimately to the direct sum decomposition of its $L^{2}-$
space. A Peter-Weyl theory for a semicomplete orthonormal set is also possible
and is here developed in this paper for compact groups. Existence of
semicomplete orthonormal sets on a compact group is proved by an explicit
construction of the standard Riemann-Lebesgue semicomplete orthonormal set.
This approach gives an insight into the role played by the $L^{2}-$ space of a
compact group, which is discovered to be just an example (indeed the largest
example for every semicomplete orthonormal set) of what is called a
prime-Parseval subspace, which we proved to be dense in the usual $L^{2}-$
space, serves as the natural domain of the Fourier transform and breaks up into
a direct-sum decomposition. This paper essentially gives the harmonic analysis
of the prime-Parseval subsapce of a compact group corresponding to any
semicomplete orthonormal set, with an introduction to what is expected for all
connected semisimple Lie groups through the notion of a $K-$semicomplete
orthonormal set.
