

A177078


Central polygonalfree numbers.


0



3, 5, 9, 13, 15, 17, 19, 23, 25, 27, 31, 39, 41, 43, 45, 47, 51, 53, 57, 59, 61, 65, 69, 71, 73, 75, 81, 83, 85, 89, 93, 95, 97, 101, 103, 107, 109, 113, 115, 117, 123, 125, 127, 129, 131, 135, 139, 141, 149, 151, 153, 155, 157, 159, 163, 167, 169, 171, 173, 177, 179
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OFFSET

1,1


COMMENTS

Positive integers not multiples of any element except 1 of A000124. Complement of multiples of central polygonal numbers (other than multiples of 1, which are all integers) = {2, 4, 6, 7, 8, 10, 12, 14, 16, 18, 20, 21, 22, 24, ...}. This is to squarefree numbers (A005117), i.e., numbers that are not divisible by a square greater than 1, as central polygonal numbers (A000124) are to squares (A000290).


LINKS

Table of n, a(n) for n=1..61.


FORMULA

{a(n)} = {k such that k>0 and for no j>1 does A000124(j)  k} = {k such that k>0 and for no j>1 does (j(j+1)/2 + 1)  k}.


EXAMPLE

Construct by sieve. Start with all integers from 2 through N. Eliminate all multiples of A000124(1) = 2. Now we have all odd numbers. It is redundant to eliminate all multiples of A000124(2) = 4 as those were all eliminated in the previous step. Now eliminate all multiples of A000124(3) = 7. Now eliminate all multiples of A000124(4) = 11. Continue through eliminating all multiples of A000124 elements > 1 through N and one has all elements of this new sequence through N^2.


MAPLE

A000124 := proc(n) n*(n+1)/2+1 ; end proc:
isA177078 := proc(n) for j from 1 do if n mod A000124(j) = 0 then return false; elif A000124(j)> n then return true; end if; end do: end proc:
for n from 2 to 300 do if isA177078(n) then printf("%d, ", n) ; end if; end do: # R. J. Mathar, May 28 2010


CROSSREFS

Cf. A000124, A000290, A005117.
Sequence in context: A161866 A102968 A180341 * A187906 A187229 A089593
Adjacent sequences: A177075 A177076 A177077 * A177079 A177080 A177081


KEYWORD

easy,nonn


AUTHOR

Jonathan Vos Post, May 02 2010


EXTENSIONS

9 inserted, 29 removed, 37 removed and sequence extended by R. J. Mathar, May 28 2010


STATUS

approved



