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

 Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A099071 Composite numbers n such that the concatenation of all nonprime natural numbers up to n in decreasing order is prime. 3
 4, 6, 8, 9, 26, 1752 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The terms of this sequence are composite terms of the sequence A099070 with same order. Next term is greater than 6000 and the prime corresponding to the next term has more than 20000 digits. Number of digits of primes corresponding to the six known terms of the sequence are respectively 2,3,4,5,29 & 5010. LINKS C. Rivera, Primes by Listing. EXAMPLE 26 is in the sequence because 26 is composite; nonprimes up to 26 are 1,4,6,8,9,10,12,14,15,16,18,20,21,22,24,25,26 and 26252422212018161514121098641 is prime. MATHEMATICA Do[If[ !PrimeQ[n]&&PrimeQ[(v={}; Do[If[ !PrimeQ[n+1-j], v=Join[v, IntegerDigits[n+1-j]]], {j, n}]; FromDigits[v])], Print[n]], {n, 6013}] cnpQ[n_]:=PrimeQ[FromDigits[Flatten[IntegerDigits/@Select[Range[n, 1, -1], !PrimeQ[#]&]]]]; Select[Range[1800], !PrimeQ[#]&&cnpQ[#]&] (* Harvey P. Dale, Jul 19 2020 *) CROSSREFS Cf. A099070, A100003, A046284. Sequence in context: A161732 A066307 A287198 * A156673 A073866 A202260 Adjacent sequences:  A099068 A099069 A099070 * A099072 A099073 A099074 KEYWORD base,more,nonn,nice AUTHOR Farideh Firoozbakht, Nov 06 2004 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.

Last modified December 2 19:12 EST 2020. Contains 338891 sequences. (Running on oeis4.)