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 A057138 Add (n mod 10)*10^(n-1) to the previous term, with a(0) = 0. 5
 0, 1, 21, 321, 4321, 54321, 654321, 7654321, 87654321, 987654321, 987654321, 10987654321, 210987654321, 3210987654321, 43210987654321, 543210987654321, 6543210987654321, 76543210987654321, 876543210987654321 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Original definition: "Concatenate next digit at left hand end." This is misleading, since the concatenation of 0 yields the same term (leading zeros vanish), but upon the next concatenation of 1, the 0 reappears - except for a(1), which according to that description should equal a(1)=10: It is surprising that in this only case where the 0 is indeed present, it disappears upon left-concatenation of the digit 1! - M. F. Hasler, Jan 13 2013 From Hieronymus Fischer, Jan 23 2013: (Start) A definition which is also consistent is: Start with terms 0 and 1 and then concatenate the next digit at left hand end. If the next digit is a zero, keep this zero in mind so that the then following digit is a 1 preceding a 0. The sequence terms are the terms of A057137 in reversed digit order. Based on this understanding, the anomaly for the indices 0 and 1 where the terms are 0 and 1 instead of 0 and 10 (what one would expect) becomes self-explaining. Also, the special behavior when the zero digit is encountered becomes clear. Examples: a(3) = 321 = Reversal(A057137(3)), a(10) = 987654321 = Reversal(A057137(10)) = Reversal(1234567890). (End) LINKS Hieronymus Fischer, Table of n, a(n) for n = 0..200 FORMULA a(n) = a(n-1) + 10^(n-1)*n - 10^n*floor(n/10) = A057139(n) mod 10^n. a(n) = floor(((q/(10^10 - 1)) + q mod 10^(n mod 10))*10^(10*floor(n/10))), where q = 987654321. - Hieronymus Fischer, Jan 03 2013 G.f.: x(1-10(10x)^9 + 9(10x)^10)/((1-x) (1-10x)^2 (1-(10x)^10)). - Robert Israel, Jun 21 2017 MAPLE ListTools:-PartialSums([seq((k mod 10)*10^(k-1), k=0..40)]); # Robert Israel, Jun 21 2017 MATHEMATICA Join[{c = 0}, Table[c = c + Mod[n, 10]*10^(n - 1), {n, 18}]] (* T. D. Noe, Jan 30 2013 *) PROG (PARI) a(n)=sum(i=0, n, i%10*10^(i-1)) \\ M. F. Hasler, Jan 13 2013 CROSSREFS Alternative progression for n >= 10 compared with A000422 and A014925. Cf. A057137 for reverse. Sequence in context: A113531 A069572 A260487 * A104759 A138793 A014925 Adjacent sequences:  A057135 A057136 A057137 * A057139 A057140 A057141 KEYWORD base,easy,nonn AUTHOR Henry Bottomley, Aug 12 2000 EXTENSIONS Better definition from M. F. Hasler, Jan 13 2013 STATUS approved

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Last modified October 14 18:28 EDT 2019. Contains 328022 sequences. (Running on oeis4.)