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A068679
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Numbers which yield a prime whenever a 1 is inserted anywhere in them (including at the beginning or end).
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17
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1, 3, 7, 13, 31, 49, 63, 81, 91, 99, 103, 109, 117, 123, 151, 181, 193, 213, 231, 279, 319, 367, 427, 459, 571, 601, 613, 621, 697, 721, 801, 811, 951, 987, 1113, 1117, 1131, 1261, 1821, 1831, 1939, 2101, 2149, 2211, 2517, 2611, 3151, 3219, 4011, 4411, 4519, 4887, 5031, 5361, 6231, 6487, 6871, 7011, 7209, 8671, 9141, 9801, 10051
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OFFSET
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1,2
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COMMENTS
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If R(p) = (10^p -1)/9 is a prime then {(10^(p-1) -1}/9 belongs to this sequence.
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LINKS
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EXAMPLE
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123 belongs to this sequence as the numbers 1123, 1213, 1231 obtained by inserting a 1 in all possible ways are all primes.
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MATHEMATICA
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d[n_]:=IntegerDigits[n]; ins[n_]:=FromDigits/@Table[Insert[d[n], 1, k], {k, Length[d[n]]+1}]; Select[Range[10060], And@@PrimeQ/@ins[#] &] (* Jayanta Basu, May 20 2013 *)
Select[Range[11000], AllTrue[FromDigits/@Table[Insert[ IntegerDigits[ #], 1, n], {n, IntegerLength[#]+1}], PrimeQ]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 16 2020 *)
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PROG
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(Python)
from sympy import isprime
if isprime(10*n+1):
s = str(n)
for i in range(len(s)):
if not isprime(int(s[:i]+'1'+s[i:])):
break
else:
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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EXTENSIONS
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More terms from Eli McGowan (ejmcgowa(AT)mail.lakeheadu.ca), Apr 11 2002
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STATUS
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approved
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