A069278 - OEIS

%I #32 Aug 31 2024 18:05:45

%S 131072,196608,294912,327680,442368,458752,491520,663552,688128,

%T 720896,737280,819200,851968,995328,1032192,1081344,1105920,1114112,

%U 1146880,1228800,1245184,1277952,1492992,1507328,1548288,1605632,1622016

%N 17-almost primes (generalization of semiprimes).

%C Product of 17 not necessarily distinct primes.

%C Divisible by exactly 17 prime powers (not including 1).

%C For n = 1..2628 a(n)=2*A069277(n). - _Zak Seidov_, Jun 25 2017

%H D. W. Wilson, <a href="/A069278/b069278.txt">Table of n, a(n) for n = 1..10000</a>

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/AlmostPrime.html">Almost Prime.</a>

%F Product p_i^e_i with Sum e_i = 17.

%t Select[Range[2*10^6],PrimeOmega[#]==17&] (* _Harvey P. Dale_, Sep 28 2016 *)

%o (PARI) k=17; start=2^k; finish=2000000; v=[] for(n=start,finish, if(bigomega(n)==k,v=concat(v,n))); v

%o (Python)

%o from math import isqrt, prod

%o from sympy import primerange, integer_nthroot, primepi

%o def A069278(n):

%o     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     def f(x): return int(n+x-sum(primepi(x//prod(c[1] for c in a))-a[-1][0] for a in g(x,0,1,1,17)))

%o     def bisection(f,kmin=0,kmax=1):

%o         while f(kmax) > kmax: kmax <<= 1

%o         while kmax-kmin > 1:

%o             kmid = kmax+kmin>>1

%o             if f(kmid) <= kmid:

%o                 kmax = kmid

%o             else:

%o                 kmin = kmid

%o         return kmax

%o     return bisection(f) # _Chai Wah Wu_, Aug 31 2024

%Y 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), this sequence (r = 17), A069279 (r = 18), A069280 (r = 19), A069281 (r = 20). - _Jason Kimberley_, Oct 02 2011

%K nonn

%O 1,1

%A _Rick L. Shepherd_, Mar 13 2002