A112141 Product of the first n semiprimes.

4, 24, 216, 2160, 30240, 453600, 9525600, 209563200, 5239080000, 136216080000, 4495130640000, 152834441760000, 5349205461600000, 203269807540800000, 7927522494091200000, 364666034728195200000, 17868635701681564800000, 911300420785759804800000

Offset

1,1

Comments

Semiprime analog of primorial (A002110). Equivalent for product of what A062198 is for sum.

Links

T. D. Noe, Table of n, a(n) for n = 1..100

Formula

a(n) = Product_{i=1..n} A001358(i).

A001222(a(n)) = 2*n.

Example

a(10) = 4*6*9*10*14*15*21*22*25*26 = 136216080000, the product of the first 10 semiprimes.
From Gus Wiseman, Dec 06 2020: (Start)
The sequence of terms together with their prime signatures begins:
                        4: (2)
                       24: (3,1)
                      216: (3,3)
                     2160: (4,3,1)
                    30240: (5,3,1,1)
                   453600: (5,4,2,1)
                  9525600: (5,5,2,2)
                209563200: (6,5,2,2,1)
               5239080000: (6,5,4,2,1)
             136216080000: (7,5,4,2,1,1)
            4495130640000: (7,6,4,2,2,1)
          152834441760000: (8,6,4,2,2,1,1)
         5349205461600000: (8,6,5,3,2,1,1)
       203269807540800000: (9,6,5,3,2,1,1,1)
      7927522494091200000: (9,7,5,3,2,2,1,1)
    364666034728195200000: (10,7,5,3,2,2,1,1,1)
  17868635701681564800000: (10,7,5,5,2,2,1,1,1)
(End)

Maple

A112141 := proc(n)
    mul(A001358(i), i=1..n) ;
end proc:
seq(A112141(n), n=1..10) ; # R. J. Mathar, Jun 30 2020

Mathematica

NextSemiPrime[n_, k_: 1] := Block[{c = 0, sgn = Sign[k]}, sp = n + sgn; While[c < Abs[k], While[ PrimeOmega[sp] != 2, If[sgn < 0, sp--, sp++]]; If[sgn < 0, sp--, sp++]; c++]; sp + If[sgn < 0, 1, -1]]; f[n_] := Times @@ NestList[ NextSemiPrime@# &, 2^2, n - 1]; Array[f, 18] (* Robert G. Wilson v, Jun 13 2013 *)
FoldList[Times, Select[Range[30], PrimeOmega[#]==2&]] (* Gus Wiseman, Dec 06 2020 *)

Prog

(PARI) a(n)=my(v=vector(n), i, k=3); while(i<n, if(bigomega(k++)==2, v[i++]=k)); prod(i=1, n, v[i]) \\ Charles R Greathouse IV, Apr 04 2013
(Python)
from sympy import factorint
def aupton(terms):
    alst, k, p = [], 1, 1
    while len(alst) < terms:
        if sum(factorint(k).values()) == 2:
            p *= k
            alst.append(p)
        k += 1
    return alst
print(aupton(18)) # Michael S. Branicky, Aug 31 2021

Crossrefs

Partial sums of semiprimes are A062198.

First differences of semiprimes are A065516.

A000040 lists primes, with partial products A002110 (primorials).

A000142 lists factorials, with partial products A000178 (superfactorials).

A001358 lists semiprimes, with partial products A112141 (this sequence).

A005117 lists squarefree numbers, with partial products A111059.

A006881 lists squarefree semiprimes, with partial products A339191.

A101048 counts partitions into semiprimes (restricted: A338902).

A320655 counts factorizations into semiprimes.

A338898/A338912/A338913 give the prime indices of semiprimes, with product/sum/difference A087794/A176504/A176506.

A338899/A270650/A270652 give the prime indices of squarefree semiprimes, with product/sum/difference A339361/A339362/A338900.

Cf. A001222, A084126, A084127, A115392, A168472, A320732, A320892.

Sequence in context: A162314 A369723 A323869 * A077555 A166881 A334602

Adjacent sequences: A112138 A112139 A112140 * A112142 A112143 A112144

Keyword

easy,nonn

Author

Jonathan Vos Post, Nov 28 2005

Status

approved