Let M k(N) be the space of weight k, level N weakly holomorphic modular forms with poles only at the cusp at 1. We explicitly construct a canonical basis for M] k(N) for N 2 {8, 9, 16, 25}, and show that many of the Fourier coecients of the basis elements in M] 0(N) are divisible by high powers of the prime dividing the level N. Additionally, we show that these basis elements satisfy a Zagier duality property, and extend Grin’s results on congruences in level 1 to levels 2, 3, 4, 5, 7, 8, 9, 16, and 25.
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