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A007606
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Take 1, skip 2, take 3, etc.
(Formerly M3241)
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11
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1, 4, 5, 6, 11, 12, 13, 14, 15, 22, 23, 24, 25, 26, 27, 28, 37, 38, 39, 40, 41, 42, 43, 44, 45, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 137, 138
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listen;
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OFFSET
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1,2
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COMMENTS
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List the natural numbers: 1, 2, 3, 4, 5, 6, 7, ... . Keep the first number (1), delete the next two numbers (2, 3), keep the next three numbers (4, 5, 6), delete the next four numbers (7, 8, 9, 10) and so on.
Numbers k with the property that the smallest Dyck path of the symmetric representation of sigma(k) has a central valley. (Cf. A237593.) - Omar E. Pol, Aug 28 2018
The values of k such that, in a listing of all congruence classes of positive integers, the k-th congruence class contains k. Here the class r mod m (with r in {1,...,m}) precedes the class r' mod m' (with r' in {1,...,m'}) iff m<m' or r<r'. Cf. A360418. - James Propp, Feb 10 2023
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REFERENCES
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C. Dumitrescu & V. Seleacu, editors, Some Notions and Questions in Number Theory, Vol. I, Erhus Publ., Glendale, 1994.
R. Honsberger, Mathematical Gems III, M.A.A., 1985, p. 177.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
F. Smarandache, Properties of Numbers, 1972.
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LINKS
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FORMULA
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a(n) = n + m*(m+1) where m = floor(sqrt(n-1)). - Klaus Brockhaus, Mar 26 2004
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EXAMPLE
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Written as an irregular triangle in which the row lengths are the odd numbers the sequence begins:
1;
4, 5, 6;
11, 12, 13, 14, 15;
22, 23, 24, 25, 26, 27, 28;
37, 38, 39, 40, 41, 42, 43, 44, 45;
56, 57, 58, 59, 60, 61, 62 , 63, 64, 65, 66;
79, 80, 81, 82 , 83, 84, 85, 86, 87, 88, 89, 90, 91;
106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120;
...
Right border gives A000384, n >= 1.
(End)
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MATHEMATICA
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Flatten[ Table[i, {j, 1, 17, 2}, {i, j(j - 1)/2 + 1, j(j + 1)/2}]] (* Robert G. Wilson v, Mar 11 2004 *)
Join[{1}, Flatten[With[{nn=20}, Range[#[[1]], Total[#]]&/@Take[Thread[ {Accumulate[ Range[nn]]+1, Range[nn]}], {2, -1, 2}]]]] (* Harvey P. Dale, Jun 23 2013 *)
With[{nn=20}, Take[TakeList[Range[(nn(nn+1))/2], Range[nn]], {1, nn, 2}]]//Flatten (* Harvey P. Dale, Feb 10 2023 *)
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PROG
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(PARI) for(n=1, 66, m=sqrtint(n-1); print1(n+m*(m+1), ", "))
(Haskell)
a007606 n = a007606_list !! (n-1)
a007606_list = takeSkip 1 [1..] where
takeSkip k xs = take k xs ++ takeSkip (k + 2) (drop (2*k + 1) xs)
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CROSSREFS
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KEYWORD
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nonn,tabf,nice,easy
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AUTHOR
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STATUS
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approved
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