A169730 - OEIS

%I #6 Jun 02 2016 12:53:44

%S 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,

%T 17,21,38,24,25,19,25,43,44,20,29,49,31,1,37,31,38,35,58,29,37,67,41,

%U 68,51,8,47,77,49,46,58,49,7,82,51,59,47,51,83,11,53,66,92

%N 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.

%e 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.

%p A006881 := proc(n)

%p     option remember;

%p     if n = 1 then

%p         6;

%p     else

%p         for a from procname(n-1)+1 do

%p             if numtheory[bigomega](a)=2 and issqrfree(a) then

%p                 return a;

%p             end if;

%p         end do:

%p     end if;

%p end proc:

%p A169730 := proc(n)

%p     local p,k,pm,pk;

%p     p := numtheory[factorset](A006881(n)) ;

%p     pm := op(1,p) ;

%p     pk := op(2,p) ;

%p     k := numtheory[pi](pk) ;

%p     m := numtheory[pi](pm) ;

%p     abs(pm*k-pk*m) ;

%p end proc:

%p seq(A169730(n),n=1..72) ; # _R. J. Mathar_, Jun 02 2016

%Y Cf. A006881.

%K nonn,less

%O 1,5

%A _Juri-Stepan Gerasimov_, Apr 28 2010

%E Corrected by _R. J. Mathar_, Jun 02 2016