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A103534
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Concatenations of pairs of primes that differ by 1000.
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3
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131013, 191019, 311031, 611061, 971097, 1031103, 1091109, 1511151, 1631163, 1811181, 1931193, 2231223, 2291229, 2771277, 2831283, 3071307, 3671367, 3731373, 4091409, 4331433, 4391439, 4871487, 4991499, 5231523, 5711571
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Integers in this sequence can never be prime, as they are all multiples of 3. They can be semiprimes, as with the values for which the smaller prime P is prime(i) for i = 25, 36, 63, 93, 127...
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FORMULA
| a(n) = Concatenate(P, P+1000) iff P prime and P+1000 prime.
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EXAMPLE
| 1811181 is in this sequence because 181 is prime, 181+1000 = 1181 is prime and those two primes are concatenated. The result is not itself a prime, as 1811181 = 3 x 23 x 26249. But it does happen to read the same backwards (palindromatic) and upside-down.
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CROSSREFS
| Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719, A103523.
Sequence in context: A202935 A013893 A013693 * A165478 A051331 A075972
Adjacent sequences: A103531 A103532 A103533 * A103535 A103536 A103537
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KEYWORD
| base,easy,nonn
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AUTHOR
| Jonathan Vos Post (jvospost3(AT)gmail.com), Mar 22 2005
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