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).
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
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