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A052056 Numbers k such that k! starts with its largest prime substring. 1
2, 4, 6, 7, 9, 10, 15, 16, 20, 21, 23, 25, 30, 35, 43, 78, 102, 105, 132, 138, 151, 189, 202, 215, 219, 233, 241, 264, 320, 334, 349, 352, 367, 386, 433, 458, 520, 583, 779, 885, 905, 1068, 1078, 1131, 1149, 1198, 1271, 1276, 1314, 1503, 1623, 1646, 1903, 1962, 2053 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
Michael S. Branicky, Python program
EXAMPLE
16 is a term because 16! = {209227}89888000 and its largest prime substring 209227 starts from the left.
PROG
(Python)
from sympy import isprime
def starts_with_lps(n): # see link for faster version
s = str(n)
ss = (s[i:j] for i in range(len(s)) for j in range(i+1, len(s)+1))
lps = max((u for u in (int(t) for t in ss) if isprime(u)), default=0)
return lps > 0 and s.startswith(str(lps))
def afind():
k, fk = 1, 1
while True:
if starts_with_lps(fk):
print(k, end=", ")
k += 1
fk *= k
afind() # Michael S. Branicky, Dec 31 2021
CROSSREFS
Sequence in context: A050099 A184333 A184411 * A356754 A187974 A193715
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, Jan 15 2000
EXTENSIONS
More terms from Sean A. Irvine, Feb 16 2011
Offset changed to 1 by Jon E. Schoenfield, Oct 17 2019
a(38)-a(49) from Michael S. Branicky, Dec 31 2021
a(50)-a(55) from Michael S. Branicky, May 31 2023
STATUS
approved

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Last modified March 28 14:38 EDT 2024. Contains 371254 sequences. (Running on oeis4.)