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A340539
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a(n) is the least prime that is the concatenation of two n-digit primes, and such that the concatenation of the same primes in the other order is also prime or 0 if no such prime exists.
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1
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37, 1123, 101107, 10091789, 1000710709, 100003100363, 10000031000303, 1000001910002521, 100000007100010321, 10000000071000000349, 1000000001910000000799, 100000000003100000009939, 10000000000391000000012387, 1000000000003710000000034573, 100000000000031100000000014113
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OFFSET
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1,1
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COMMENTS
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Conjecture: a(n) > 0 and for n > 1 the first n digits of a(n) = A003617(n). - Chai Wah Wu, Jan 13 2021
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LINKS
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EXAMPLE
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For n=4, 1009, 1789, 10091789 and 17891009 are all prime.
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MAPLE
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f:= proc(d) local P, a, b;
a:= prevprime(10^(d-1));
do
a:= nextprime(a);
if a > 10^d then return FAIL fi;
b:= prevprime(10^(d-1));
do
b:= nextprime(b);
if b > 10^d then break fi;
if isprime(10^d*a+b) and isprime(10^d*b+a) then return 10^d*a+b fi;
od od:
FAIL
end proc:
f(1):= 37:
map(f, [$1..20]);
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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