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A121608
Primes that can be written as concatenation of two composite numbers in decimal representation.
5
89, 109, 149, 229, 269, 349, 359, 389, 409, 421, 433, 439, 449, 457, 463, 487, 491, 499, 509, 569, 659, 677, 691, 709, 769, 809, 821, 827, 829, 839, 857, 859, 863, 877, 881, 887, 919, 929, 977, 991, 1009, 1021, 1033, 1039, 1049, 1051, 1063, 1069, 1087, 1091
OFFSET
1,1
LINKS
EXAMPLE
A000040(169) = 1009 = 100*10+9 = A002808(74)*10+A002808(4), therefore 1009 is a term: a(41) = 1009;
A000040(172) = 1021 = 10*100+21 = A002808(5)*100+A002808(12), therefore 1021 is a term: a(42) = 1021.
PROG
(Python)
from sympy import isprime
def comp(s): i=int(s); return s[0]!='0' and i > 1 and not isprime(i)
def ok(n):
s = str(n)
for i in range(1, len(s)):
if comp(s[:i]) and comp(s[i:]) and isprime(int(s)): return True
print([m for m in range(1092) if ok(m)]) # Michael S. Branicky, Feb 27 2021
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Reinhard Zumkeller, Aug 10 2006
STATUS
approved