  In the 5D theory in which only 3 generation right-handed neutrinos are in the
bulk, the neutrino flavor mixings and the mass spectrum can be constructed
through the seesaw mechanism. The 5D seesaw is easily calculated just by a
replacement of the
  Majorana mass eigenvalues, M_i, by 2 M_*tan(h)[\pi RM_i] (M_*: 5D Planck
scale,
  R: compactification radius). The 5D features appear when the bulk mass, which
induces the 4D Majorana mass, is the same as the compactification scale or
larger than it. Depending on the type of bulk mass, the seesaw scales of the 3
generations are strongly split (the tan-function case) or degenerate (the
tanh-function case). In the split case, the seesaw enhancement is naturally
realized. The single right-handed neutrino dominance works in a simple setup,
and some specific mass textures, which are just assumptions in the 4D setup,
can be naturally obtained in
  5 dimensions. The degenerate case is also useful for a suitable neutrino
flavor structure.
