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A064455
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a(2n) = 3n, a(2n-1) = n.
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19
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1, 3, 2, 6, 3, 9, 4, 12, 5, 15, 6, 18, 7, 21, 8, 24, 9, 27, 10, 30, 11, 33, 12, 36, 13, 39, 14, 42, 15, 45, 16, 48, 17, 51, 18, 54, 19, 57, 20, 60, 21, 63, 22, 66, 23, 69, 24, 72, 25, 75, 26, 78, 27, 81, 28, 84, 29, 87, 30, 90, 31, 93, 32, 96, 33, 99, 34, 102, 35, 105, 36, 108
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,2
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COMMENTS
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Number of ON cells at generation n of 1-D CA defined by Rule 54. - N. J. A. Sloane, Aug 09 2014
Given a(1) = 1, for all n > 1, a(n) is the least positive integer not equal to a(n-1) such that the arithmetic mean of the first n terms is an integer. The sequence of arithmetic means of the first 1, 2, 3, ..., terms is 1, 2, 2, 3, 3, 4, 4, ... (A004526 disregarding its first three terms). - Rick L. Shepherd, Aug 20 2013
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LINKS
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FORMULA
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a(n) = (1/2)*n*(-1)^n + n + (1/4)*(-1)^(n+1) + 1/4. - Stephen Crowley, Aug 10 2009
G.f.: x*(1+3*x) / ( (x-1)^2*(1+x)^2 ). - R. J. Mathar, Mar 30 2011
a(n) = n + a(n-1) for even n.
a(n) = (n^2-3n+2) mod (2n-1) for n>2. - Jim Singh, Oct 31 2018
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EXAMPLE
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a(13) = a(2*7 - 1) = 7, a(14) = a(2*7) = 21.
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MAPLE
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if type(n, 'even') then
3*n/2 ;
else
(n+1)/2 ;
end if;
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MATHEMATICA
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Table[ If[ EvenQ[n], 3n/2, (n + 1)/2], {n, 1, 70} ]
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PROG
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(ARIBAS): maxarg := 75; for n := 1 to maxarg do if n mod 2 = 1 then write((n+1) div 2, " ") else write((n div 2)*3, " "); end; end; .
(PARI) { for (n=1, 1000, if (n%2, a=(n + 1)/2, a=3*n/2); write("b064455.txt", n, " ", a) ) } \\ Harry J. Smith, Sep 14 2009
(PARI) a(n)=if(n<3, 2*n-1, ((n-1)*(n-2))%(2*n-1)) \\ Jim Singh, Oct 14 2018
(Haskell)
import Data.List (transpose)
a064455 n = n + if m == 0 then n' else - n' where (n', m) = divMod n 2
a064455_list = concat $ transpose [[1 ..], [3, 6 ..]]
(Magma) [(1/2)*n*(-1)^n+n+(1/4)*(-1)^(n+1)+1/4: n in [1..80]]; // Vincenzo Librandi, Aug 10 2014
(GAP) a:=[];; for n in [1..75] do if n mod 2 = 0 then Add(a, 3*n/2); else Add(a, (n+1)/2); fi; od; a; # Muniru A Asiru, Oct 28 2018
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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