A106415 Smallest number beginning with 5 that is the product of exactly n distinct primes.

5, 51, 506, 510, 5610, 51870, 510510, 50169210, 504894390, 50012172210, 503520607590, 50001975553530, 501601785815130, 50073188107872930, 5000089945706645790, 50617203592231346070, 5000858931483646541310

Offset

1,1

Links

Chai Wah Wu, Table of n, a(n) for n = 1..45

Example

a(4) = 510 = 2*3*5*17.

Prog

(Python)
from itertools import count
from math import prod, isqrt
from sympy import primerange, integer_nthroot, primepi, primorial
def A106415(n):
    if n == 1: return 5
    def g(x, a, b, c, m): yield from (((d, ) for d in enumerate(primerange(b+1, isqrt(x//c)+1), a+1)) if m==2 else (((a2, b2), )+d for a2, b2 in enumerate(primerange(b+1, integer_nthroot(x//c, m)[0]+1), a+1) for d in g(x, a2, b2, c*b2, m-1)))
    def f(x): return int(sum(primepi(x//prod(c[1] for c in a))-a[-1][0] for a in g(x, 0, 1, 1, n)))
    for l in count(len(str(primorial(n)))-1):
        kmin, kmax = 5*10**l-1, 6*10**l-1
        mmin, mmax = f(kmin), f(kmax)
        if mmax>mmin:
            while kmax-kmin > 1:
                kmid = kmax+kmin>>1
                mmid = f(kmid)
                if mmid > mmin:
                    kmax, mmax = kmid, mmid
                else:
                    kmin, mmin = kmid, mmid
    return kmax # Chai Wah Wu, Sep 12 2024

Crossrefs

Cf. A077326-A077334, A106411-A106419, A106421-A106429.

Sequence in context: A317782 A342407 A195211 * A212819 A041040 A088320

Adjacent sequences: A106412 A106413 A106414 * A106416 A106417 A106418

Keyword

base,nonn

Author

Ray Chandler, May 02 2005

Status

approved