What’s the smallest nonprime that is not obviously nonprime? For Ramanujan the answer would have been in the thousands, but for the rest of us, I think it’s 91. The sequence of nonobvious nonprimes runs 91, 119, 133, 143, 161, 187,….

Here’s my reasoning. If a number is divisible by 2, 3, or 5, that’s obvious, and if it is 11 times a single digit, that’s obvious too. Also, perfect squares are pretty familiar. This leaves us with 7•13, 7•17, 7•19, 11•13, 7•23, 11•17,….

[14 May 2016]