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.

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A099861 a(n) = (2*n-1)-st composite number: a bisection of A002808.

Original entry on oeis.org

4, 8, 10, 14, 16, 20, 22, 25, 27, 30, 33, 35, 38, 40, 44, 46, 49, 51, 54, 56, 58, 62, 64, 66, 69, 72, 75, 77, 80, 82, 85, 87, 90, 92, 94, 96, 99, 102, 105, 108, 111, 114, 116, 118, 120, 122, 124, 126, 129, 132, 134, 136, 140, 142, 144, 146, 148, 152, 154, 156, 159, 161
Offset: 1

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Author

N. J. A. Sloane, Nov 19 2004

Keywords

Examples

			a(1) = 4 is the first composite number.

Links

Crossrefs

Cf. A002808, A099862, A175228.

Programs

  • Maple
    b:=proc(n) if isprime(n)=true then else n fi end: B:=[seq(b(n),n=2..250)]: seq(B[2*m-1],m=1..75); # Emeric Deutsch, Dec 09 2004
  • Mathematica
    Partition[Select[Range[200], CompositeQ], 2][[All, 1]] (* Jean-François Alcover, Mar 22 2023 *)
  • Python
    from sympy import composite
def A099861(n): return composite((n<<1)-1) # Chai Wah Wu, Nov 14 2024

Formula

a(n) = A175228(n+1). - A.H.M. Smeets, Aug 19 2019

Extensions

More terms from Emeric Deutsch, Dec 09 2004

A175227 Sequence of the remaining nonprimes out of sieve of type: {step 1: starting with the sequence of positive integers A000027; step 2: delete every prime number A000040; step 3: delete every prime(1)-th number greater than the prime(1) from the remaining sequence; step 4: delete every prime(2)-th number greater than the prime(2) from the remaining sequence; etc. for prime(k)-th numbers for k = 3, 4, 5, ...}.

Original entry on oeis.org

1, 4, 8, 14, 16, 22, 30, 33, 40, 49, 54, 56, 62, 77, 80, 92, 94, 99, 111, 116, 118, 132, 144, 152, 154, 166, 174, 182
Offset: 1

Views

Author

Jaroslav Krizek, Mar 07 2010

Keywords

Comments

Sequence of step 1: A000027, sequences of step 2: delete A000040 = primes, remaining A018252 = nonprimes, sequences of step 3: delete A099862 = {6, 9, 12, 15, 18, 21, 24, 26, 28, 32, ...}, remaining A175228 = {1, 4, 8, 10, 14, 16, 20, 22, 25, 27, ...}, sequences of step 4: delete A175229 = {10, 20, 27, 35, 44, 51, 58, 66, 75, 82, ...}, remaining A175230 = {1, 4, 8, 14, 16, 22, 25, 30, 33, 38, ...}, sequences of step 5: delete sequence = {25, 46, 64, 85, 102, 122, 140, 159, 176, 196, ...}, remaining sequence = {1, 4, 8, 14, 16, 22, 30, 33, 38, 40, ...}, ...
Showing 1-2 of 2 results.

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

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