login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A340487 a(n) is the least prime that is the concatenation of two n-digit primes, or 0 if there are none. 2
23, 1117, 101107, 10091021, 1000710181, 100003100129, 10000031000171, 1000001910000349, 100000007100000541, 10000000071000000349, 1000000001910000000319, 100000000003100000000063, 10000000000391000000000903, 1000000000003710000000000259, 100000000000031100000000000403 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: a(n) > 0 and the first n digits of a(n) = A003617(n). - Chai Wah Wu, Jan 13 2021

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..500 (n = 1..380 from Robert Israel)

EXAMPLE

a(3) = 101107 is prime and the concatenation of the two 3-digit primes 101 and 107.

MAPLE

f:= proc(d) local P, a, b;

  a:= prevprime(10^(d-1));

  do

    a:= nextprime(a);

    if a > 10^d then return FAIL fi;

    b:= prevprime(10^(d-1));

    do

      b:= nextprime(b);

      if b > 10^d then break fi;

      if isprime(10^d*a+b) then return 10^d*a+b fi;

  od od:

  FAIL

end proc:

f(1):= 23:

map(f, [$1..20]);

CROSSREFS

Cf. A003617.

Sequence in context: A126741 A273477 A202656 * A321569 A061063 A100768

Adjacent sequences:  A340484 A340485 A340486 * A340488 A340489 A340490

KEYWORD

nonn,base

AUTHOR

J. M. Bergot and Robert Israel, Jan 10 2021

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
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 16 10:30 EDT 2021. Contains 345056 sequences. (Running on oeis4.)