OFFSET
1,1
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..10000
EXAMPLE
The sequence of all relatively prime pairs (columns) begins:
1 1 1 1 2 1 1 2 1 3 1 1 2 3 1 1 2 1 4 1 3 1 2 1 3
1 2 3 4 3 5 6 5 7 4 8 9 7 5 10 11 9 12 5 13 7 14 11 15 8
MATHEMATICA
Select[Range[100], And[PrimeOmega[#]==2, GCD@@PrimePi/@If[#==1, {}, FactorInteger[#]][[All, 1]]==1]&]
With[{nn=40}, Join[{4}, Take[Prime[#[[1]]]Prime[#[[2]]]&/@Select[Subsets[ Range[ nn], {2}], CoprimeQ@@#&]//Union, Floor[Prime[nn]/2]]]] (* Harvey P. Dale, Jan 04 2021 *)
PROG
(PARI) ok(n)={my(f=factor(n)); bigomega(f)==2 && gcd(apply(primepi, f[, 1]))==1} \\ Andrew Howroyd, Oct 26 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Sep 06 2018
STATUS
approved