A080682 19-smooth numbers: numbers whose prime divisors are all <= 19.

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 44, 45, 48, 49, 50, 51, 52, 54, 55, 56, 57, 60, 63, 64, 65, 66, 68, 70, 72, 75, 76, 77, 78, 80, 81, 84, 85, 88, 90, 91, 95, 96, 98, 99, 100

Offset

1,2

Links

William A. Tedeschi, Table of n, a(n) for n = 1..10000

Formula

Sum_{n>=1} 1/a(n) = Product_{primes p <= 19} p/(p-1) = (2*3*5*7*11*13*17*19)/(1*2*4*6*10*12*16*18) = 323323/55296. - Amiram Eldar, Sep 22 2020

Mathematica

mx = 120; Sort@ Flatten@ Table[ 2^i*3^j*5^k*7^l*11^m*13^n*17^o*19^p, {i, 0, Log[2, mx]}, {j, 0, Log[3, mx/2^i]}, {k, 0, Log[5, mx/(2^i*3^j)]}, {l, 0, Log[7, mx/(2^i*3^j*5^k)]}, {m, 0, Log[11, mx/(2^i*3^j*5^k*7^l)]}, {n, 0, Log[13, mx/(2^i*3^j*5^k*7^l*11^m)]}, {o, 0, Log[17, mx/(2^i*3^j*5^k*7^l*11^m*13^n)]}, {p, 0, Log[19, mx/(2^i*3^j*5^k*7^l*11^m*13^n*17^o)]}] (* Robert G. Wilson v, Jan 19 2016 *)
Select[Range[100], Max[FactorInteger[#][[All, 1]]]<20&] (* Harvey P. Dale, Sep 20 2018 *)

Prog

(PARI) test(n)= {m=n; forprime(p=2, 19, while(m%p==0, m=m/p)); return(m==1)}
for(n=1, 200, if(test(n), print1(n", ")))
(PARI) list(lim, p=19)=if(p==2, return(powers(2, logint(lim\1, 2)))); my(v=[], q=precprime(p-1), t=1); for(e=0, logint(lim\=1, p), v=concat(v, list(lim\t, q)*t); t*=p); Set(v) \\ Charles R Greathouse IV, Apr 16 2020
(Magma) [n: n in [1..100] | PrimeDivisors(n) subset PrimesUpTo(19)]; // Bruno Berselli, Sep 24 2012
(Python)
import heapq
from itertools import islice
from sympy import primerange
def agen(p=19): # generate all p-smooth terms
    v, oldv, h, psmooth_primes, = 1, 0, [1], list(primerange(1, p+1))
    while True:
        v = heapq.heappop(h)
        if v != oldv:
            yield v
            oldv = v
            for p in psmooth_primes:
                heapq.heappush(h, v*p)
print(list(islice(agen(), 72))) # Michael S. Branicky, Nov 20 2022
(Python)
from sympy import integer_log
def A080682(n):
    def bisection(f, kmin=0, kmax=1):
        while f(kmax) > kmax: kmax <<= 1
        while kmax-kmin > 1:
            kmid = kmax+kmin>>1
            if f(kmid) <= kmid:
                kmax = kmid
            else:
                kmin = kmid
        return kmax
    def g(x, m): return sum((x//3**i).bit_length() for i in range(integer_log(x, 3)[0]+1)) if m==3 else sum(g(x//(m**i), prevprime(m))for i in range(integer_log(x, m)[0]+1))
    def f(x): return n+x-g(x, 19)
    return bisection(f, n, n) # Chai Wah Wu, Sep 16 2024

Crossrefs

For p-smooth numbers with other values of p, see A003586, A051037, A002473, A051038, A080197, A080681, A080683.

Sequence in context: A004830 A081330 A359415 * A182049 A038770 A342591

Adjacent sequences: A080679 A080680 A080681 * A080683 A080684 A080685

Keyword

easy,nonn

Author

Cino Hilliard, Mar 02 2003

Status

approved