OFFSET
1,1
LINKS
Michael S. Branicky, Table of n, a(n) for n = 1..10000
EXAMPLE
210 = 2*3*5*7 is a term.
130169 = 13*17*19*31 is a term.
PROG
(Python)
from sympy import factorint
def ok(n):
f = factorint(n)
return len(f) == sum(f.values()) == 4 and len(set([len(str(p)) for p in f])) == 1
print([k for k in range(144000) if ok(k)]) # Michael S. Branicky, Oct 08 2024
(Python)
from math import prod
from sympy import primerange
from itertools import count, combinations, islice
def bgen(d): # generator of terms that are products of d-digit primes
primes, out = list(primerange(10**(d-1), 10**d)), set()
for t in combinations(primes, 4): out.add(prod(t))
yield from sorted(out)
def agen(): # generator of terms
for d in count(1): yield from bgen(d)
print(list(islice(agen(), 34))) # Michael S. Branicky, Oct 08 2024
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Paul Duckett, Oct 07 2024
EXTENSIONS
Terms corrected by Michael S. Branicky, Oct 08 2024
STATUS
approved