The Classification of Flats in$${\boldsymbol {PG}}({\bf 9,2})$$ which are External to the Grassmannian$${\cal G}_{\bf 1,4,2}$$

  • Authors:
  • Ron Shaw;Johannes G. Maks;Neil A. Gordon

  • Affiliations:
  • Department of Mathematics, University of Hull, Hull, UK HU6 7RX;Department of Mathematics, Delft University of Technology, Delf, The Netherlands 2600 GA;Department of Computer Science, University of Hull, Hull, UK HU6 7RX

  • Venue:
  • Designs, Codes and Cryptography
  • Year:
  • 2005

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Abstract

Constructions are given of different kinds of flats in the projective space $$PG(9,2)={\mathbb P}(\wedge^{2}V(5,2))$$ which are external to the Grassmannian $${\cal G}_{\bf 1,4,2}$$ of lines of PG(4,2). In particular it is shown that there exist precisely two GL(5,2)-orbits of external 4-flats, each with stabilizer group 驴31:5. (No 5-flat is external.) For each k=1,2,3, two distinct kinds of external k-flats are simply constructed out of certain partial spreads in PG(4,2) of size k+2. A third kind of external plane, with stabilizer 驴23:(7:3), is also shown to exist. With the aid of a certain `key counting lemma驴, it is proved that the foregoing amounts to a complete classification of external flats.