We establish an isomorphism between the center of the twisted Heisenberg category and the subalgebra Γ of the symmetric functions generated by odd power sums. We give a graphical description of the factorial Schur Q-functions and inhomogeneous power sums as closed diagrams in the twisted Heisenberg category, and show that the bubble generators of the center correspond to two sets of generators of Γ which encode data related to up/down transition functions on the Schur graph. Finally, we describe an action of the trace of the twisted Heisenberg category, the W -algebra W − ⊂ W 1+∞ , on Γ.
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