A246805
Lexicographically earliest sequence of distinct terms such that, when i
1, 3, 4, 7, 19, 31, 67, 391, 583, 4549, 917467, 6777061, 86794921, 1421517037, 171234891469
Offset: 1
Examples
The following concatenations are prime: - j=2: a(1) U a(2)=13, a(2) U a(1)=31 - j=3: a(3) U a(1)=41, a(3) U a(2)=43 - j=4: a(1) U a(4)=17, a(4) U a(1)=71, a(2) U a(4)=37, a(4) U a(2)=73, a(3) U a(4)=47 - j=5: a(5) U a(1)=191, a(5) U a(2)=193, a(3) U a(5)=419, a(4) U a(5)=719, a(5) U a(4)=197 - j=6: a(1) U a(6)=131, a(6) U a(1)=311, a(2) U a(6)=331, a(6) U a(2)=313, a(3) U a(6)=431, a(6) U a(4)=317, a(5) U a(6)=1931, a(6) U a(5)=3119
Links
- Paul Tek, PARI program for this sequence
Programs
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PARI
See Link section.
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Python
from sympy import isprime from itertools import islice def c(s, slst): return all(isprime(int(s+t)) or isprime(int(t+s)) for t in slst) def agen(): slst, an, mink = [], 1, 2 while True: yield an; slst.append(str(an)); an += 1 while not c(str(an), slst): an += 1 print(list(islice(agen(), 10))) # Michael S. Branicky, Oct 17 2022
Extensions
a(15) from Michael S. Branicky, Nov 07 2022
Comments