121212121

Appears in sequences

• Rows of Pascal's triangle mod 3.at n=8A006940
• Two-bell analog of A028355.at n=12A028359
• Decimal expansion of a(n) is given by the first n terms of the periodic sequence with initial period 1,2.at n=8A037487
• Catalan paths: numbers starting with 1 and ending with 1 where each digit is positive and adjacent digits differ by 1.at n=9A079215
• Palindromes p such that 5p + 1 is also a palindrome.at n=28A083833
• Palindromes n such that 6n + 1 is also a palindrome.at n=28A083835
• 1 concatenated with n 21's.at n=4A137466
• a(n) is the smallest number not already present that permits the cyclic repetition of the path 1,2 of the digits in the sequence.at n=15A165300
• Underline all even terms; concatenations of underlined terms and of non-underlined terms both equal the concatenation of the entire sequence. This is the lexicographically earliest such sequence without duplicate terms and with an even digit among the first two terms.at n=23A307312