122122

Appears in sequences

• Decimal expansion of a(n) is given by the first n terms of the periodic sequence with initial period 1,2,2.at n=5A037551
• k th digit of a(n) = number of different digits within 2 places of k (not including k).at n=17A039987
• n expressed in Fibonacci binary-like number system using only 1's and 2's.at n=35A108960
• Sorted list of strings that can be obtained by starting with 12 and repeatedly doubling any substring in place.at n=28A130838
• Numbers such that each decimal digit d (for all d in the range 1 <= d <= 9) occurs only in runs of length exactly d.at n=16A140057
• Numbers for which the sum of reciprocals of square of digits is an integer.at n=14A182452
• List of imprimitive words over the alphabet {1,2}.at n=14A213972
• Reversed shortest (x+1,2x)-code of n.at n=20A233137
• Non-primitive words on {1,2,3}.at n=23A239018
• Number of ascent sequences of length n with exactly nine flat steps.at n=5A242162
• Number of set partitions of [n] into exactly six blocks where sizes of distinct blocks are coprime.at n=8A280884
• Consider the Post tag system described in A284116 (but adapted to the alphabet {1,2}) ; sequence lists the words that terminate in the empty word.at n=47A289675
• Take n-th string over {1,2} in lexicographic order and apply the Watanabe tag system {00, 1011} described in A291067 (but adapted to the alphabet {1,2}) just once.at n=47A291072
• a(1) = 1; a(2) = 2; for n > 2, a(n) is the least number > a(n-1) whose decimal representation is uniquely the concatenation of the decimal representations of two distinct earlier terms.at n=39A336528