login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A034808 Concatenation of 'prevprime(n) and n' is a prime. 13
3, 9, 37, 39, 51, 63, 87, 89, 111, 117, 123, 153, 157, 163, 173, 177, 183, 207, 211, 213, 217, 219, 239, 249, 257, 263, 267, 269, 273, 277, 279, 289, 321, 323, 327, 333, 337, 339, 343, 359, 369, 379, 407, 423, 439, 441, 459, 471, 473, 477, 479, 489, 497, 513 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Since there are primes in the sequence, and concat(p,p) = p*(10^x+1) is always composite, it is clear that here the variant 2 (A151799(n) < n) of the prevprime function is used, rather than the variant 1 (A007917(n) <= n). - M. F. Hasler, Sep 09 2015

LINKS

Table of n, a(n) for n=0..53.

EXAMPLE

n=333 -> previous prime is 331, thus '331333' is a prime.

MATHEMATICA

coQ[n_]:=PrimeQ[FromDigits[Flatten[IntegerDigits[{NextPrime[n, -1], n}]]]]; Select[Range[3, 513], coQ[#]&] (* Jayanta Basu, May 30 2013 *)

CROSSREFS

Cf. A034809-A034821.

Cf. A151799, A007917.

Sequence in context: A135474 A279121 A076964 * A018398 A197309 A149021

Adjacent sequences:  A034805 A034806 A034807 * A034809 A034810 A034811

KEYWORD

nonn,base

AUTHOR

Patrick De Geest, Oct 15 1998.

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified February 19 22:04 EST 2018. Contains 299357 sequences. (Running on oeis4.)