A112228 Product of the first n (semiprimes - 1).

3, 15, 120, 1080, 14040, 196560, 3931200, 82555200, 1981324800, 49533120000, 1585059840000, 52306974720000, 1778437140480000, 65802174197760000, 2500482619514880000, 112521717878169600000, 5401042458152140800000, 270052122907607040000000, 14582814637010780160000000

Offset

1,1

Comments

Semiprime analog of A005867.

Links

Table of n, a(n) for n=1..19.

Formula

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

Example

a(10) = (4-1)*(6-1)*(9-1)*(10-1)*(14-1)*(15-1)*(21-1)*(22-1)*(25-1)*(26-1) = 3*5*8*9*13*14*20*21*24*25 = 49533120000, the product of the first 10 (semiprimes minus one).

Mathematica

FoldList[Times, Select[Range[50], PrimeOmega[#]==2&]-1] (* Harvey P. Dale, Jan 21 2023 *)

Prog

(PARI) A001358(n)={
    if(n==1,
        return(4),
        for(a=A001358(n-1)+1, oo,
            if (bigomega(a)==2,
                return(a)
            ) ;
        ) ;
    ) ;
}
A112228(n)={
    prod(i=1, n, A001358(i)-1)
} /* R. J. Mathar, Mar 09 2012 */

Crossrefs

Cf. A001358, A005867.

Sequence in context: A006454 A225115 A303353 * A260511 A093571 A093570

Adjacent sequences: A112225 A112226 A112227 * A112229 A112230 A112231

Keyword

easy,nonn

Author

Jonathan Vos Post, Nov 28 2005

Extensions

More terms from Jason Yuen, Oct 06 2025

Status

approved