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 A167503 Positions of nonzero digits in this sequence, where the terms are written in base 3 (and concatenated). 4
 1, 2, 10, 12, 20, 21, 100, 101, 102, 112, 121, 122, 201, 202, 210, 211, 212, 220, 221, 222, 1000, 1001, 1002, 1011, 1012, 1021, 1022, 1100, 1102, 1110, 1111, 1112, 1120, 1121, 1122, 1200, 1202, 1210, 1211, 1212, 1220, 1221, 1222, 2010, 2020, 2021, 2101 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS This sequence lists the positions of nonzero digits in the sequence, where the terms are written in base 3 and concatenated, such that the least significant digit (units) of a(n) is followed by the most significant digit of a(n+1). LINKS EXAMPLE The list cannot start with 0 (cf. A167500), so the first digit of the sequence is nonzero, whence a(1)=1. The next nonzero digit of this sequence will be the most significant digit (m.s.d.) of a(2), necessarily nonzero, thus a(2)=2. For the same reason, a(3) = 3[10] = 10[3]. (All terms are written in base 3.) Thus the 4th digit is zero, followed by the m.s.d. of the next term which is nonzero, thus a(4)=5. Terms of the sequence:_ 1,2,10,12,20,21,100,101 Position of the digits: 1 2 34 56 78 9A BCD EFG (A=10,...) Thus the numbers 4,8,12,13,15,..., which give the positions of digits '0', are not in this sequence. PROG (PARI) base(n, b=3, s=1) = { my( a=[ n%b ]); while( 0=n & for(i=a[n-1]+1, #b, b[i] & (a=concat(a, i)) & break); #a

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