A216983 The integers sieved by 7, 5, 3, and 2.

7, 1, 2, 3, 2, 5, 3, 7, 2, 3, 5, 1, 3, 1, 7, 5, 2, 1, 3, 1, 5, 7, 2, 1, 3, 5, 2, 3, 7, 1, 5, 1, 2, 3, 2, 7, 3, 1, 2, 3, 5, 1, 7, 1, 2, 5, 2, 1, 3, 7, 5, 3, 2, 1, 3, 5, 7, 3, 2, 1, 5, 1, 2, 7, 2, 5, 3, 1, 2, 3, 7, 1, 3, 1, 2, 5, 2, 7, 3, 1, 5, 3, 2, 1, 7, 5, 2

Offset

1,1

Comments

A number is tested for the following in order - the first test passed determines a(n):

Is n mod 7 == 1? If so, write 7.

Is n mod 5 == 1? If so, write 5.

Is n mod 3 == 1? If so, write 3.

Is n mod 2 == 1? If so, write 2.

Write 1.

Period 210. - Charles R Greathouse IV, Sep 22 2012

An example of an inverted sieve. Usually we would sieve 2, 3, 5, 7 which gives 2, 1, 2, 3, 2, 5, 2, 7, 2, 3, 2, 1. - Jon Perry, Sep 24 2012

Links

Table of n, a(n) for n=1..87.

Example

4 is not 1 mod 7 or 1 mod 5 but is 1 mod 3, so a(4) = 3.

Mathematica

Table[If[Mod[n, 7] == 1, 7, If[Mod[n, 5] == 1, 5, If[Mod[n, 3] == 1, 3, If[Mod[n, 2] == 1, 2, 1]]]], {n, 100}] (* T. D. Noe, Sep 25 2012 *)
Table[Which[Mod[n, 7]==1, 7, Mod[n, 5]==1, 5, Mod[n, 3]==1, 3, Mod[n, 2]==1, 2, True, 1], {n, 90}] (* Harvey P. Dale, Mar 04 2016 *)

Prog

(JavaScript) for (i=1; i<90; i++)
if (i%7==1) document.write("7, ");
else if (i%5==1) document.write("5, ");
else if (i%3==1) document.write("3, ");
else if (i%2==1) document.write("2, ");
else document.write("1, ");
(C++)
template <typename T> std::string PrintSequnce_A216983(const T &max)
{
  std::string strSeq;
  for(T i = 1; i < max; ++i)
  {
    if (i%7==1) strSeq+="7";
    else if (i%5==1) strSeq+="5";
    else if (i%3==1) strSeq+="3";
    else if (i%2==1) strSeq+="2";
    else strSeq+="1";
    if(i<max-1)
      strSeq+=", ";
   }
   return strSeq;
} // Martin Ettl, Oct 08 2012

Crossrefs

Sequence in context: A134898 A371946 A176440 * A021588 A224998 A222216

Adjacent sequences: A216980 A216981 A216982 * A216984 A216985 A216986

Keyword

nonn,easy

Author

Jon Perry, Sep 21 2012

Status

approved