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A216165
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Composite numbers and 1 which yield a prime whenever a 1 is inserted anywhere in them, including at the beginning or end.
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5
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1, 49, 63, 81, 91, 99, 117, 123, 213, 231, 279, 319, 427, 459, 621, 697, 721, 801, 951, 987, 1113, 1131, 1261, 1821, 1939, 2101, 2149, 2211, 2517, 2611, 3151, 3219, 4011, 4411, 4887, 5031, 5361, 6231, 6487, 7011, 7209, 8671, 9141, 9801, 10051, 10161, 10281
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OFFSET
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1,2
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LINKS
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EXAMPLE
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7209 is not prime but 72091, 72019, 72109, 71209 and 17209 are all primes.
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MAPLE
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with(numtheory);
local a, b, c, i, n, ok;
for n from 1 to q do
if not isprime(n) then
a:=n; b:=0; while a>0 do b:=b+1; a:=trunc(a/10); od; a:=n; ok:=1;
for i from 0 to b do c:=a+9*10^i*trunc(a/10^i)+10^i*x;
if not isprime(c) then ok:=0; break; fi;
od;
if ok=1 then print(n); fi;
fi;
od; end:
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MATHEMATICA
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Join[{1}, Select[Range[11000], CompositeQ[#]&&AllTrue[FromDigits/@ Table[ Insert[ IntegerDigits[#], 1, i], {i, IntegerLength[#]+1}], PrimeQ]&]] (* Harvey P. Dale, Mar 24 2017 *) (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Mar 24 2017 *)
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PROG
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(Magma) [n: n in [1..11000] | not IsPrime(n) and forall{m: t in [0..#Intseq(n)] | IsPrime(m) where m is (Floor(n/10^t)*10+1)*10^t+n mod 10^t}]; // Bruno Berselli, Sep 03 2012
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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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