Quintic forms over p-adic fields

We prove that a quintic form in 26 variables defined over a p-adic field K always has a non trivial zero over K if the residue class field of K has at least 47 elements. This is in agreement with the theorem of Ax-Kochen which states that a homogeneous form of degree d in d² + 1 variables defined over Q_p has a nontrivial Q_p-rational zero if p is sufficiently large. The Ax-Kochen theorem gives no results on the bound for p. For d= 1, 2, 3 it has been known for a long time that there is a nontrivial Q_p-rational zero for all values of p. For d=4, Terjanian gave an example of a form in 18 variables over Q₂ having no nontrivial Q₂-rational zero. This is the first result which gives an effective bound for the case d= 5.