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A089922
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Largest odd number in the reverse concatenation of the first n consecutive odd numbers when that concatenation is prime.
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2
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OFFSET
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3,1
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COMMENTS
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These numbers are rare. For the case of the reverse concatenation of the consecutive numbers 1,2,3,... I have only found 828180...54321. Certainly, we can reduce the trials by noting that 2/3 of the numbers formed this way are multiples of 3 for the consecutive numbers and 1/3 of the odd consecutive numbers is a multiple of 3 but I do not think that will help much using PARI.
The number corresponding to 54335431...7531 has 10313 digits. The next term, if it exists is greater than 13000. [From Dmitry Kamenetsky, Feb 21 2009]
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LINKS
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Table of n, a(n) for n=3..7.
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EXAMPLE
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31 is the reverse concatenation of the consecutive odd numbers 1 and 3. 31 is
prime. 817815813...531 is the reverse concatenation of the odd numbers 1 to 817.
817815813...531 is a probable prime.
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PROG
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(PARI) revprime2(n) = { y=1; forstep(x=3, n, 2, y=concat(Str(x), Str(y)); z=eval(y); if(ispseudoprime(z), print(x", "z)) ) }
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CROSSREFS
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Cf. original concatenation in A038395. [From Dmitry Kamenetsky, Feb 21 2009]
Sequence in context: A006270 A139888 A079594 * A041279 A054699 A102267
Adjacent sequences: A089919 A089920 A089921 * A089923 A089924 A089925
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KEYWORD
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nonn,base
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AUTHOR
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Cino Hilliard (hillcino368(AT)gmail.com), Jan 11 2004
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EXTENSIONS
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Edited by T. D. Noe, Oct 30 2008
Added a new term 5433. This number is a probable prime with 20 iterations of Miller-Rabin test. Dmitry Kamenetsky, Feb 21 2009
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STATUS
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approved
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