A147819 := proc(n) if n = 1 then 2; else for a from procname(n-1)+1 do if isprime(a-1) or isprime(a+1) then if a <> 3 then return a; end if; end if; end do ; end if; end proc: A168389 := proc(n) numtheory[pi](A147819(n)) ; end proc: seq(A168389(n), n=1..100) ; # R. J. Mathar, Dec 03 2009