161

This number is a composite.

161 divided by seventy-two is a commonly used rational approximation of the square root of seven minus two. [Honaker]

The smallest semiprime that yields a palindromic prime when turned upside down. [Ondrejka]

The smallest number not divisible by 2 or 3 and not a perfect power such that it is not the largest number less than n of maximal order mod n for any positive integer n. [Luen]

The smallest phisemiprime: a semiprime formed by the initial digits of the decimal expansion of Golden Ratio. The sequence begins 161, 1618, 161803. 16180339887498948482, 16180339887498948482045868343 and so on. [Gupta]

161 is the smallest semiprime which produces 5 more (distinct) semiprimes through concatenating the prime factors (in increasing order) over and over again , i.e: 161 = 7 * 23 --> 723 = 3 * 241 --> 3241 = 7 * 463 --> 7463 = 17 * 439 --> 17439 = 3 * 5813 --> 35813 = 59 * 607 . [Rivera]