Which expression does not factor?

The sum of squares m² + 1 does not factor into polynomials with integer coefficients. The other choices factor: m³ + 1 = (m + 1)(m² − m + 1), m³ − 1 = (m − 1)(m² + m + 1), and m² − 1 = (m − 1)(m + 1).

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