A124269 3-almost primes indexed by primes.

12, 18, 27, 30, 50, 63, 75, 78, 102, 124, 130, 164, 172, 175, 190, 231, 246, 258, 279, 286, 292, 332, 345, 369, 404, 418, 425, 430, 435, 452, 524, 539, 574, 578, 606, 610, 638, 652, 663, 692, 722, 725, 775, 782, 795, 801, 854, 906, 916, 927, 938, 963, 969

Offset: 1

Author

Jonathan Vos Post, Oct 23 2006

Keywords

• easy
• nonn

Comments

Primes indexed by 3-almostprimes = A124268. prime(3almostprime(n)) - 3almostprime(prime(n)) = A124270. See also A106349 Primes indexed by semiprimes. See also A106350 Semiprimes indexed by primes. See also A122824 Prime(semiprime(n)) - semiprime(prime(n)). Commutator [A000040,A001358] at n.

Examples

			a(1) = 3almostprime(prime(1)) = 3almostprime(2) = 12 = 2^2 * 3.
a(2) = 3almostprime(prime(2)) = 3almostprime(3) = 18 = 2 * 3^2.
a(3) = 3almostprime(prime(3)) = 3almostprime(5) = 27 = 3^3.
		

Links

• Giovanni Resta, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000040, A014612, A122824, A124268, A124270.

Programs

Maple

From R. J. Mathar, Oct 15 2010: (Start)
A014612 := proc(n) option remember; if n = 1 then 8; else for a from procname(n-1)+1 do if numtheory[bigomega](a) = 3 then return a; end if; end do; end if; end proc:
A124269 := proc(n) A014612(ithprime(n)) ; end proc: seq(A124269(n),n=1..80) ; (End)

Mathematica

p3 = Select[Range[1000], PrimeOmega[#] == 3 &]; p3[[Prime@ Range@ PrimePi@ Length@ p3]] (* Giovanni Resta, Jun 13 2016 *)

Formula

a(n) = 3almostprime(prime(n)) = A014612(A000040(n)).

Extensions

More terms from R. J. Mathar, Oct 15 2010