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The vacuum Verma module has a level-one null vector
Statement
For every , the vacuum Verma module has a nonzero singular vector at grade one. Its Shapovalov norm is zero, it lies in the radical, and . This is an algebraic null-vector example; it asserts no positivity or Hilbert-space structure.
Facts & Assumptions
Given: The algebraic module and its normalized form.
Virasoro Verma modules and contravariant forms: for , , and ; the normalized form is contravariant and its radical is the unique maximal proper submodule.
PBW basis and grades of Virasoro Verma modules: each grade has its partition basis and is the nonzero basis vector for the unique partition of grade one.
Proof
Put . It is nonzero by [F3] and has grade one because by [F1]. For , . For every , , since . Thus is singular. Also , so it is not a second grade-zero vector.
Contravariance gives , because . If is homogeneous of grade , then , , and contravariance under gives . Thus is orthogonal to every grade . Grade one is spanned by by [F3], and its self-pairing is zero, so is orthogonal to the whole algebraic direct sum and lies in the radical. At general highest weight , the grade-one Gram matrix is the one-entry matrix , so at its determinant is zero for every central charge . A zero norm here is a bilinear-form statement and uses no positivity assumption.