5

This number is a prime.

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5 requires 5 steps to reach 1 in the 3x + 1 (or Collatz) Problem. Paul Erdős said about the Collatz problem: "Mathematics is not yet ready for such confusing, troubling, and hard problems." He offered $500 for its solution. (Lagarias, 1985)

Submitted: 1999-09-02 23:40:00; Last Modified: 2012-04-02 19:24:59.