Tight t-Designs and Squarefree Integers

The authors prove, using a variety of number-theoretical methods, that tight t-designs in the projective spaces FPn of ‘lines’ through the origin in Fn+1 (F = ℂ, or the quarternions H) satisfy t ⩽ 5. Such a design is a generalisation of a combinatorial t-design. It is known that t ⩽ 5 in the cases F = ℝ , O (the octonions) and that t ⩽ 11 for tight spherical t-designs; hence the author's result essentially completes the classification of tight t-designs in compact connected symmetric spaces of rank 1.