A179695 Numbers of the form p^3*q^2*r^2 where p, q, and r are distinct primes.

1800, 2700, 3528, 4500, 5292, 8712, 9800, 12168, 12348, 13068, 18252, 20808, 24200, 24500, 25992, 31212, 33075, 33800, 34300, 38088, 38988, 47432, 47916, 55125, 57132, 57800, 60500, 60552, 66248, 69192, 72200, 77175, 79092, 81675, 84500

Offset: 1

Author

Vladimir Joseph Stephan Orlovsky, Jul 24 2010

Keywords

• nonn

Comments

Subsequence of A225228. - Reinhard Zumkeller, May 03 2013

Links

• T. D. Noe, Table of n, a(n) for n = 1..1000
• Will Nicholes, List of prime signatures, 2010.
• Index to sequences related to prime signature.

Crossrefs

Cf. A050326, A225228.

Cf. A085548, A085541, A085964, A085965, A085967.

Programs

Mathematica

f[n_]:=Sort[Last/@FactorInteger[n]]=={2,2,3}; Select[Range[10^5], f]
f[n_]:={Times@@(n^{2,2,3}),Times@@(n^{2,3,2}),Times@@(n^{3,2,2})}; Module[ {nn=20},Select[Flatten[f/@Subsets[Prime[Range[nn]],{3}]],#<= 72*Prime[ nn]^2&]]//Union (* Harvey P. Dale, Jul 05 2019 *)

PARI

list(lim)=my(v=List(),t1,t2);forprime(p=2, (lim\36)^(1/3), t1=p^3;forprime(q=2, sqrt(lim\t1), if(p==q, next);t2=t1*q^2;forprime(r=q+1, sqrt(lim\t2), if(p==r,next);listput(v,t2*r^2)))); vecsort(Vec(v)) \\ Charles R Greathouse IV, Jul 24 2011

Python

from math import isqrt
from sympy import primepi, primerange, integer_nthroot
def A179695(n):
    def bisection(f,kmin=0,kmax=1):
        while f(kmax) > kmax: kmax <<= 1
        kmin = kmax >> 1
        while kmax-kmin > 1:
            kmid = kmax+kmin>>1
            if f(kmid) <= kmid:
                kmax = kmid
            else:
                kmin = kmid
        return kmax
    def f(x): return n+x+sum((t:=primepi(s:=isqrt(y:=isqrt(x//r**3))))+(t*(t-1)>>1)-sum(primepi(y//k) for k in primerange(1, s+1)) for r in primerange(integer_nthroot(x,3)[0]+1))+sum(primepi(isqrt(x//p**5)) for p in primerange(integer_nthroot(x,5)[0]+1))-primepi(integer_nthroot(x,7)[0])
    return bisection(f,n,n) # Chai Wah Wu, Mar 27 2025

Formula

A050326(a(n)) = 5. - Reinhard Zumkeller, May 03 2013

Sum_{n>=1} 1/a(n) = P(2)^2*P(3)/2 - P(3)*P(4)/2 - P(2)*P(5) + P(7) = 0.0032578591481263202818..., where P is the prime zeta function. - Amiram Eldar, Mar 07 2024