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 A103534 Concatenations of pairs of primes that differ by 1000. 3
 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; text; internal format)
 OFFSET 1,1 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... LINKS FORMULA a(n) = Concatenate(P, P+1000) iff P prime and P+1000 prime. 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. MATHEMATICA 10001#+1000&/@Select[Prime[Range[150]], PrimeQ[#+1000]&] (* Harvey P. Dale, Sep 01 2017 *) CROSSREFS Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719, A103523. Sequence in context: A202935 A013893 A013693 * A239610 A165478 A051331 Adjacent sequences:  A103531 A103532 A103533 * A103535 A103536 A103537 KEYWORD base,easy,nonn AUTHOR Jonathan Vos Post, Mar 22 2005 STATUS approved

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Last modified January 21 12:58 EST 2022. Contains 350477 sequences. (Running on oeis4.)