191

This number is a prime.

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The lesser in the largest known twin prime pair of the form n*2^(n-1) +/- 1. [Honaker]

Question: What is the only prime whose square has five different digits that contain an even square digit in the middle with the first and last pairs being squares as well? Answer: 191. [Friend and Honaker]

191 is the smallest prime that is a semiprime when turned upside down. [Honaker]