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A215419
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Primes that remain prime when a single digit 3 is inserted between any two consecutive digits or as the leading or trailing digit.
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25
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7, 11, 17, 31, 37, 73, 271, 331, 359, 373, 673, 733, 1033, 2297, 3119, 3461, 3923, 5323, 5381, 5419, 6073, 6353, 9103, 9887, 18289, 23549, 25349, 31333, 32933, 33349, 35747, 37339, 37361, 37489, 47533, 84299, 92333, 93241, 95093, 98491, 133733, 136333, 139333
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OFFSET
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1,1
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LINKS
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EXAMPLE
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18289 is prime and also 182893, 182839, 182389, 183289, 138289, 318289.
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MAPLE
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local a, b, c, d, i, n, ok;
for n from 1 to q do
a:=ithprime(n); b:=0; while a>0 do b:=b+1; a:=trunc(a/10); od;
a:=ithprime(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(ithprime(n)); fi;
od; end:
# Alternative:
filter:= proc(n) local L, d, k, M;
if not isprime(n) then return false fi;
L:= convert(n, base, 10);
d:= nops(L);
for k from 0 to d do
M:= [seq(L[i], i=1..k), 3, seq(L[i], i=k+1..d)];
if not isprime(add(M[i]*10^(i-1), i=1..d+1)) then return false fi;
od;
true
end proc;
select(filter, [seq(i, i=3..2*10^5, 2)]); # Robert Israel, Oct 09 2017
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MATHEMATICA
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ins@n_:=Insert[IntegerDigits@n, 3, #]&/@Range@(IntegerLength@n+1);
Cases[{#, FromDigits@#&/@ins@#}&/@ Cases[Range[11, 70000], _?PrimeQ], {_, {_?PrimeQ..}}][[All, 1]] (* Hans Rudolf Widmer, Dec 21 2023 *)
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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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