cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-5 of 5 results.

A100287 First occurrence of n in A100002; the least k such that A100002(k) = n.

Original entry on oeis.org

1, 2, 5, 9, 15, 25, 31, 43, 61, 67, 87, 103, 123, 139, 169, 183, 219, 241, 259, 301, 331, 361, 391, 447, 463, 511, 553, 589, 643, 687, 723, 783, 819, 867, 931, 979, 1027, 1099, 1179, 1227, 1309, 1347, 1393, 1479, 1539, 1603, 1699, 1759, 1863, 1909, 2019, 2029
Offset: 1

Views

Author

T. D. Noe, Nov 11 2004

Keywords

Comments

Also, the first number that is crossed off at stage n in the Flavius sieve (A000960). - N. J. A. Sloane, Nov 21 2004
The sequence appears to grow roughly like 0.7825*n^2. Note that for n>2, the second occurrence of n in A100002 is at a(n)+1.
Equals main diagonal of triangle A101224, which is defined by the process starting with column 1: A101224(n,1) = n^2-n+1 for n>=1 and continuing with: A101224(n,k) = (n-k+1)*floor( (A101224(n,k-1) - 1)/(n-k+1) ) for k>1 until k=n. I.e., a(n) = A101224(n,n). - Paul D. Hanna, Dec 01 2004

Links

Crossrefs

Cf. A000960, A099259, A100002, A101224.
Column 1 of A278507, column 2 of A278505 (without the initial 1-term).

Programs

  • Mathematica
    a[n_] := Fold[#2*Ceiling[#1/#2 + 1] &, 1, Reverse@Range[n - 1]]; Array[a, 30] (* Birkas Gyorgy, Feb 16 2011 *)
  • PARI
    {a(n)=local(A);for(k=1,n,if(k==1,A=n^2-n+1,A=(n-k+1)*floor((A-1)/(n-k+1))));A}

Formula

a(n) ~ Pi/4 * n^2 (via A000960). - Bill McEachen, Aug 08 2024

A000960 Flavius Josephus's sieve: Start with the natural numbers; at the k-th sieving step, remove every (k+1)-st term of the sequence remaining after the (k-1)-st sieving step; iterate.

Original entry on oeis.org

1, 3, 7, 13, 19, 27, 39, 49, 63, 79, 91, 109, 133, 147, 181, 207, 223, 253, 289, 307, 349, 387, 399, 459, 481, 529, 567, 613, 649, 709, 763, 807, 843, 927, 949, 1009, 1093, 1111, 1189, 1261, 1321, 1359, 1471, 1483, 1579, 1693, 1719, 1807, 1899, 1933, 2023
Offset: 1

Views

Author

N. J. A. Sloane

Keywords

Comments

a(n) is never divisible by 2 or 5. - Thomas Anton, Nov 01 2018

Examples

			Start with
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ... (A000027) and delete every second term, giving
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 ... (A005408) and delete every 3rd term, giving
1 3 7 9 13 15 19 21 25 27 ... (A056530) and delete every 4th term, giving
1 3 7 13 15 19 25 27 ... (A056531) and delete every 5th term, giving
.... Continue forever and what's left is the sequence.
(The array formed by these rows is A278492.)
For n = 5, 5^2 = 25, go down to a multiple of 4 giving 24, then to a multiple of 3 = 21, then to a multiple of 2 = 20, then to a multiple of 1 = 19, so a(5) = 19.

References

  • V. Brun, Un procédé qui ressemble au crible d'Ératosthène, Analele Stiintifice Univ. "Al. I. Cuza", Iasi, Romania, Sect. Ia Matematica, 1965, vol. 11B, pp. 47-53.
  • Problems 107, 116, Nord. Mat. Tidskr. 5 (1957), 114-116.
  • N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
  • N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

Crossrefs

Cf. A056526, A056530, A056531, A100002, A190732, A003881.
Cf. A000012, A002491, A000959, A003309, A099259, A112557, A112558, A113742, A113743, A113744, A113745, A113746, A113747, A113748; A113749, A278484.
Cf. A119446 for triangle whose leading diagonal is A119447 and this sequence gives all possible values for A119447 (except A119447 cannot equal 1 because prime(n)/n is never 1).
Cf. A100617 (a left inverse), A100618.
Cf. A278169 (characteristic function).
Main diagonal of A278492, leftmost column of A278505, positions of zeros in A278528 & A278529.

Programs

  • Haskell
    a000960 n = a000960_list !! (n-1)
a000960_list = sieve 1 [1..] where
   sieve k (x:xs) = x : sieve (k+1) (flavius xs) where
      flavius xs = us ++ flavius vs where (u:us,vs) = splitAt (k+1) xs
-- Reinhard Zumkeller, Oct 31 2012
  • Maple
    S[1]:={seq(i,i=1..2100)}: for n from 2 to 2100 do S[n]:=S[n-1] minus {seq(S[n-1][n*i],i=1..nops(S[n-1])/n)} od: A:=S[2100]; # Emeric Deutsch, Nov 17 2004
  • Mathematica
    del[lst_, k_] := lst[[Select[Range[Length[lst]], Mod[ #, k] != 0 &]]]; For[k = 2; s = Range[2100], k <= Length[s], k++, s = del[s, k]]; s
f[n_] := Fold[ #2*Ceiling[ #1/#2 + 1] &, n, Reverse@Range[n - 1]]; Array[f, 60] (* Robert G. Wilson v, Nov 05 2005 *)
  • PARI
    a(n)=local(A=n,D);for(i=1,n-1,D=n-i;A=D*ceil(A/D+1));return(A) \\ Paul D. Hanna, Oct 10 2005
  • Python
    def flavius(n):
    L = list(range(1,n+1));j=2
    while j <= len(L):
        L = [L[i] for i in range(len(L)) if (i+1)%j]
        j+=1
    return L
flavius(100)
# Robert FERREOL, Nov 08 2015

Formula

Let F(n) = number of terms <= n. Andersson, improving results of Brun, shows that F(n) = 2 sqrt(n/Pi) + O(n^(1/6)). Hence a(n) grows like Pi*n^2 / 4.
To get n-th term, start with n and successively round up to next 2 multiples of n-1, n-2, ..., 1 (compare to Mancala sequence A002491). E.g.: to get 11th term: 11->30->45->56->63->72->80->84->87->90->91; i.e., start with 11, successively round up to next 2 multiples of 10, 9, .., 1. - Paul D. Hanna, Oct 10 2005
As in Paul D. Hanna's formula, start with n^2 and successively move down to the highest multiple of n-1, n-2, etc., smaller than your current number: 121 120 117 112 105 102 100 96 93 92 91, so a(11) = 91, from moving down to multiples of 10, 9, ..., 1. - Joshua Zucker, May 20 2006
Or, similarly for n = 5, begin with 25, down to a multiple of 4 = 24, down to a multiple of 3 = 21, then to a multiple of 2 = 20 and finally to a multiple of 1 = 19, so a(5) = 19. - Joshua Zucker, May 20 2006
This formula arises in A119446; the leading term of row k of that triangle = a(prime(k)/k) from this sequence. - Joshua Zucker, May 20 2006
a(n) = 2*A073359(n-1) + 1, cf. link to posts on the SeqFan list. - M. F. Hasler, Nov 23 2016
a(n) = 1 + A278484(n-1). - Antti Karttunen, Nov 23 2016, after David W. Wilson's posting on SeqFan list Nov 22 2016

Extensions

More terms and better description from Henry Bottomley, Jun 16 2000
Entry revised by N. J. A. Sloane, Nov 13 2004
More terms from Paul D. Hanna, Oct 10 2005

A099264 A000960(n) - A100287(n).

Original entry on oeis.org

0, 1, 2, 4, 4, 2, 8, 6, 2, 12, 4, 6, 10, 8, 12, 24, 4, 12, 30, 6, 18, 26, 8, 12, 18, 18, 14, 24, 6, 22, 40, 24, 24, 60, 18, 30, 66, 12, 10, 34, 12, 12, 78, 4, 40, 90, 20, 48, 36, 24, 4, 132, 24, 6, 56, 40, 6, 144, 32, 20, 116, 40, 74, 40, 120, 48, 82, 66, 116, 22, 30, 96, 80, 10, 60
Offset: 1

Views

Author

N. J. A. Sloane, Nov 16 2004

Keywords

Comments

Always positive.

Crossrefs

Cf. A000960, A100002, A099264.

Extensions

More terms from David Wasserman, Feb 27 2008

A099259 A100287(n+1) - A000960(n).

Original entry on oeis.org

1, 2, 2, 2, 6, 4, 4, 12, 4, 8, 12, 14, 6, 22, 2, 12, 18, 6, 12, 24, 12, 4, 48, 4, 30, 24, 22, 30, 38, 14, 20, 12, 24, 4, 30, 18, 6, 68, 38, 48, 26, 34, 8, 56, 24, 6, 40, 56, 10, 86, 6, 2, 76, 42, 72, 48, 26, 4, 40, 52, 12, 54, 12, 8, 46, 26, 24, 72, 12, 42, 74, 40, 36, 98, 22, 18, 58
Offset: 1

Views

Author

N. J. A. Sloane, Nov 16 2004

Keywords

Comments

Always positive.

Crossrefs

Cf. A000960, A100002, A099264.

Extensions

More terms from David Wasserman, Feb 27 2008

A272800 Flavius Josephus factor of n.

Original entry on oeis.org

0, 2, 0, 2, 3, 2, 0, 2, 4, 2, 3, 2, 0, 2, 5, 2, 3, 2, 0, 2, 4, 2, 3, 2, 6, 2, 0, 2, 3, 2, 7, 2, 4, 2, 3, 2, 5, 2, 0, 2, 3, 2, 8, 2, 4, 2, 3, 2, 0, 2, 6, 2, 3, 2, 5, 2, 4, 2, 3, 2, 9, 2, 0, 2, 3, 2, 10, 2, 4, 2, 3, 2, 7, 2, 5, 2, 3, 2, 0, 2, 4, 2, 3, 2, 6, 2
Offset: 1

Views

Author

Max Barrentine, May 06 2016

Keywords

Comments

This sequence is analogous to the smallest prime factor of n. If n is a member of A000960, a(n) = 0, otherwise a(n) = the (k+1)-st step that rejects n from Flavius Josephus' sieve.
The n-values of records of this sequence are given by A100287 (see 2004 comment by Sloane).

Links

Crossrefs

Cf. A000960, A100002, A100287.

Programs

  • Sage
    # Function that returns an array of the first n terms.
def A272800(n):
    A, B, k = [0]*n, range(n), 1
    while kDanny Rorabaugh, May 13 2016
Showing 1-5 of 5 results.

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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