cp's OEIS Frontend

A069279 Products of exactly 18 primes (generalization of semiprimes).

%I A069279 #38 Feb 16 2025 08:32:46
%S A069279 262144,393216,589824,655360,884736,917504,983040,1327104,1376256,
%T A069279 1441792,1474560,1638400,1703936,1990656,2064384,2162688,2211840,
%U A069279 2228224,2293760,2457600,2490368,2555904,2985984,3014656,3096576,3211264,3244032,3317760,3342336,3440640
%N A069279 Products of exactly 18 primes (generalization of semiprimes).
%C A069279 Product of 18 not necessarily distinct primes.
%C A069279 Divisible by exactly 18 prime powers (not including 1).
%H A069279 D. W. Wilson, <a href="/A069279/b069279.txt">Table of n, a(n) for n = 1..10000</a>
%H A069279 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/AlmostPrime.html">Almost Prime.</a>
%F A069279 Product p_i^e_i with Sum e_i = 18.
%t A069279 Select[Range[31*10^5],PrimeOmega[#]==18&] (* _Harvey P. Dale_, Apr 05 2015 *)
%o A069279 (PARI) k=18; start=2^k; finish=4000000; v=[]; for(n=start,finish, if(bigomega(n)==k,v=concat(v,n))); v
%o A069279 (Python)
%o A069279 from math import prod, isqrt
%o A069279 from sympy import primerange, integer_nthroot, primepi
%o A069279 def A069279(n):
%o A069279     def g(x,a,b,c,m): yield from (((d,) for d in enumerate(primerange(b,isqrt(x//c)+1),a)) if m==2 else (((a2,b2),)+d for a2,b2 in enumerate(primerange(b,integer_nthroot(x//c,m)[0]+1),a) for d in g(x,a2,b2,c*b2,m-1)))
%o A069279     def f(x): return int(n-1+x-sum(primepi(x//prod(c[1] for c in a))-a[-1][0] for a in g(x,0,1,1,18)))
%o A069279     kmin, kmax = 1,2
%o A069279     while f(kmax) >= kmax:
%o A069279         kmax <<= 1
%o A069279     while True:
%o A069279         kmid = kmax+kmin>>1
%o A069279         if f(kmid) < kmid:
%o A069279             kmax = kmid
%o A069279         else:
%o A069279             kmin = kmid
%o A069279         if kmax-kmin <= 1:
%o A069279             break
%o A069279     return kmax # _Chai Wah Wu_, Aug 23 2024
%Y A069279 Cf. A101637, A101638, A101605, A101606.
%Y A069279 Sequences listing r-almost primes, that is, the n such that A001222(n) = r: A000040 (r = 1), A001358 (r = 2), A014612 (r = 3), A014613 (r = 4), A014614 (r = 5), A046306 (r = 6), A046308 (r = 7), A046310 (r = 8), A046312 (r = 9), A046314 (r = 10), A069272 (r = 11), A069273 (r = 12), A069274 (r = 13), A069275 (r = 14), A069276 (r = 15), A069277 (r = 16), A069278 (r = 17), this sequence (r = 18), A069280 (r = 19), A069281 (r = 20). - _Jason Kimberley_, Oct 02 2011
%K A069279 nonn
%O A069279 1,1
%A A069279 _Rick L. Shepherd_, Mar 13 2002