A169730 Write the n-th squarefree semiprime as prime(m)*prime(k). Then a(n) is the absolute value of prime(m)*k-prime(k)*m.

1, 1, 1, 1, 2, 1, 1, 7, 3, 1, 3, 8, 5, 13, 8, 14, 9, 9, 9, 19, 13, 9, 15, 16, 15, 28, 10, 29, 17, 17, 21, 38, 24, 25, 19, 25, 43, 44, 20, 29, 49, 31, 1, 37, 31, 38, 35, 58, 29, 37, 67, 41, 68, 51, 8, 47, 77, 49, 46, 58, 49, 7, 82, 51, 59, 47, 51, 83, 11, 53, 66, 92

Offset

1,5

Links

Table of n, a(n) for n=1..72.

Example

a(1)=1 because prime(1)*2-prime(2)*1=4-3=1; a(2)=1 because prime(1)*3-prime(3)*2=6-5=1.

Maple

A006881 := proc(n)
    option remember;
    if n = 1 then
        6;
    else
        for a from procname(n-1)+1 do
            if numtheory[bigomega](a)=2 and issqrfree(a) then
                return a;
            end if;
        end do:
    end if;
end proc:
A169730 := proc(n)
    local p, k, pm, pk;
    p := numtheory[factorset](A006881(n)) ;
    pm := op(1, p) ;
    pk := op(2, p) ;
    k := numtheory[pi](pk) ;
    m := numtheory[pi](pm) ;
    abs(pm*k-pk*m) ;
end proc:
seq(A169730(n), n=1..72) ; # R. J. Mathar, Jun 02 2016

Crossrefs

Cf. A006881.

Sequence in context: A301922 A144510 A143670 * A220725 A333142 A196832

Adjacent sequences: A169727 A169728 A169729 * A169731 A169732 A169733

Keyword

nonn,less

Author

Juri-Stepan Gerasimov, Apr 28 2010

Extensions

Corrected by R. J. Mathar, Jun 02 2016

Status

approved