login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 59th year, we have over 358,000 sequences, and we’ve crossed 10,300 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A034808 Concatenation of 'prevprime(n) and n' is a prime. 14
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

Harvey P. Dale, Table of n, a(n) for n = 0..1000

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 *)

Select[Range[2, 550], PrimeQ[NextPrime[#, -1]*10^IntegerLength[#]+#]&] (* Harvey P. Dale, Nov 22 2020 *)

PROG

(Python)

from sympy import isprime, prevprime

def aupto(m):

return [k for k in range(3, m+1) if isprime(int(str(prevprime(k))+str(k)))]

print(aupto(513)) # Michael S. Branicky, Mar 09 2021

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 | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 7 17:25 EST 2022. Contains 358668 sequences. (Running on oeis4.)