Projective spectrum of a ring

A scheme associated with a graded ring (cf. also Graded module). As a set of points, is a set of homogeneous prime ideals such that does not contain . The topology on is defined by the following basis of open sets: for , . The structure sheaf of the locally ringed space is defined on the basis open sets as follows: , that is, the subring of the elements of degree of the ring of fractions with respect to the multiplicative system .

The most important example of a projective spectrum is . The set of its -valued points for any field is in natural correspondence with the set of points of the -dimensional projective space over the field .

If all the rings as -modules are spanned by ( terms), then an additional structure is defined on . Namely, the covering and the units determine a Čech -cocycle on to which an invertible sheaf, denoted by , corresponds. The symbol usually denotes the -th tensor power of . There exists a canonical homomorphism , indicating the geometric meaning of the grading of the ring (see [1]). If, for example, , then corresponds to a sheaf of hyperplane sections in .

References

[1]	D. Mumford, "Lectures on curves on an algebraic surface" , Princeton Univ. Press (1966)
[2]	A. Grothendieck, "Eléments de géometrie algebrique" Publ. Math. IHES , 1–4 (1960–1967)

Comments

See also Projective scheme.

References

[a1]	R. Hartshorne, "Algebraic geometry" , Springer (1977) pp. 91