231213

Appears in sequences

• Inequivalent solutions to Langford (or Langford-Skolem) problem of arranging the numbers 1,1,2,2,3,3,...,n,n so that there is one number between the two 1's, two numbers between the two 2's, ..., n numbers between the two n's, listed by length and lexicographic order.at n=0A050998
• Base 10 weak Skolem-Langford numbers.at n=2A108116
• Base-10 super-weak Skolem-Langford numbers.at n=4A339803
• Base-10 weaker Skolem-Langford numbers.at n=0A357826