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An integer polynomial for even parity modulo a power of two

For integer ell>=1 define p_ell(Y)=(1-Y)^ell times sum_{j=0}^{ell-1} binom(ell+j-1,j)Y^j. The finite sum is the inverse of (1-Y)^ell modulo Y^ell: multiplying leaves constant term one and all degrees 1 through ell-1 zero. Thus p_ell is congruent to 1 modulo Y^ell and is divisible by (1-Y)^ell. At an even integer y, y^ell is divisible by 2^ell, so p_ell(y) is congruent to 1 modulo 2^ell. At an odd y, (1-y)^ell is divisible by 2^ell, giving residue zero. These are integer congruences, not the assertion that p_ell(y) always equals zero or one.

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Updated 2026-11-07

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