Polar harmonic Maass forms and holomorphic projection

Recently, Mertens, Ono and the third author studied mock modular analogues of Eisenstein series. Their coefficients are given by small divisor functions, and have shadows given by classical Shimura theta functions. Here, we construct a class of small divisor functions sigma(sm)(2,chi) and prove that these generate the holomorphic part of polar harmonic Maass forms of weight 3/2. To this end, we essentially compute the holomorphic projection of mixed harmonic Maass forms in terms of Jacobi polynomials, but without assuming the structure of such forms. Instead, we impose translation invariance and suitable growth conditions on the Fourier coefficients. Specializing to a certain choice of characters, we obtain an identity between sigma(sm)(2,1) and Hurwitz class numbers, and ask for more such identities. Moreover, we prove p-adic congruences of our small divisor functions when p is an odd prime. If chi is non-trivial we rewrite the generating function of sigma(sm)(2,chi) as a linear combination of Appell-Lerch sums and their first two normalized derivatives. Lastly, we offer a connection of our construction to meromorphic Jacobi forms of index - 1 and false theta functions.