1423

This number is a prime.

The smallest prime number that can be formed by concatenating the first four positive integers. [Murthy]

Joy of Thinking: The Beauty and Power of Classical Mathematical Ideas is The Teaching Company's course number 1423.

The smallest prime which is sum of composite numbers written with primes repeated twice for each term : 1423 = 22+33+55+1313 = 33+77+1313. [Capelle]

A 4-digit prime, which if repeated 4 times after number 4, yields another prime whose sum of digits is 44. [Silva]

The smallest 4-digit prime whose sum of the two leftmost digits (1 and 4) equals the sum of the two rightmost digits (2 and 3). [Forouhari]