The maximal length of real trivectors of rank seven

Trivectors of rank seven over the complex number field have irreducible length less than or equal to four. Over the reals however this is not true. An example of a real trivector of rank seven and irreducible length five is presented. Then, in the notation of Busemann and Glassco, we have N(R, 7, 3) = 5 since for any field F we always have N(F ,7, 3) ≤ 5. This paper provides the first published example where N(F, n, r) ≠ N(K, n, r) for two different fields F and K.