Shifted quasi-symmetric functions and the Hopf algebra of peak functions
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In his work on P-partitions, Stembridge defined the algebra of peak functions II, which is both a subalgebra and a retraction of the algebra of quasi-symmetric functions. We show that II is closed under coproduct, and therefore a Hopf algebra, and describe the kernel of the retraction. Billey and Haiman, in their work on Schubert polynomials, also defined a new class of quasi-symmetric functions-shifted quasi-symmetric functions-and we show that II is strictly contained in the linear span S of shifted quasi-symmetric functions. We show that E is a coalgebra, and compute the rank of the nth graded component. 2002 Elsevier Science B.V. All rights reserved.