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A089922 Largest odd number in the reverse concatenation of the first n consecutive odd numbers when that concatenation is prime. 3
3, 73, 123, 817, 5433 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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. [Dmitry Kamenetsky, Feb 21 2009]

LINKS

Table of n, a(n) for n=1..5.

EXAMPLE

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.

PROG

(PARI) revprime2(n) = { y=1; forstep(x=3, n, 2, y=concat(Str(x), Str(y)); z=eval(y); if(ispseudoprime(z), print(x, ", ")) ) }

CROSSREFS

Cf. original concatenation in A038395. [Dmitry Kamenetsky, Feb 21 2009]

Sequence in context: A139888 A271628 A079594 * A259848 A041279 A054699

Adjacent sequences:  A089919 A089920 A089921 * A089923 A089924 A089925

KEYWORD

nonn,base

AUTHOR

Cino Hilliard, Jan 11 2004

EXTENSIONS

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

STATUS

approved

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Last modified October 19 03:24 EDT 2019. Contains 328211 sequences. (Running on oeis4.)