J. Gawrylczyk, THE FRACTIONAL-GRADED EXTENSION OF THE VIRASORO ALGEBRA, Journal of physics. A, mathematical and general, 27(19), 1994, pp. 120000733-120000738
Extension of the Virasoro algebra by the infinite number of generators
with homogeneous fractional gradation and the conformal dimensions 1
< DELTA less-than-or-equal-to 2 is given. We obtain the new families o
f centrally extended, constrained algebras, describing the conformal s
eries of dimension [(k/N) + 1], N is-an-element-of N and k is-an-eleme
nt-of {1,..., N).