A108215 4-almost primes equal to the product of two successive semiprimes.

24, 54, 90, 140, 210, 315, 462, 550, 650, 858, 1122, 1190, 1330, 1482, 1794, 2254, 2499, 2805, 3135, 3306, 3596, 4030, 4485, 5106, 5698, 6314, 6970, 7310, 7482, 7917, 8463, 8742, 8930, 10070, 11766, 12765, 13570, 14042, 14399, 14762, 15006, 15867

Offset

1,1

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

a(1) = 4*6 = 24 and a(2) = 6*9 = 54 because 4, 6 and 9 are the first three successive semiprimes.

Mathematica

Select[Times@@@Partition[Select[Range[200], PrimeOmega[#]==2&], 2, 1], PrimeOmega[#]==4&] (* Harvey P. Dale, Nov 17 2011 *)

Prog

(PARI) sp=4; for(sq=6, 300, if(bigomega(sq)==2, print1(sp*sq, ", "); sp=sq)) \\ Rick L. Shepherd
(PARI) list(lim)=my(v=List(), s=sqrtint(lim\1), u=v); forprime(p=2, s\2, forprime(q=2, min(p, s\p), listput(v, p*q))); v=Set(v); for(i=2, #v, listput(u, v[i-1]*v[i])); for(k=s+1, lim\v[#v], if(bigomega(k)==2, listput(u, v[#v]*k); break)); Set(u) \\ Charles R Greathouse IV, Feb 02 2017

Crossrefs

Sequence in context: A259023 A301575 A294156 * A322609 A234238 A228876

Adjacent sequences: A108212 A108213 A108214 * A108216 A108217 A108218

Keyword

easy,nonn

Author

Giovanni Teofilatto, Jun 16 2005

Extensions

More terms from Rick L. Shepherd, Jun 16 2005

Status

approved