A280576 Primes formed from the concatenation of previousprime(n) and n.

23, 79, 3137, 3739, 4751, 6163, 8387, 8389, 109111, 113117, 113123, 151153, 151157, 157163, 167173, 173177, 181183, 199207, 199211, 211213, 211217, 211219, 233239, 241249, 251257, 257263, 263267, 263269, 271273, 271277, 277279, 283289, 317321, 317323, 317327

Offset: 1

Author

K. D. Bajpai, Jan 05 2017

Keywords

• nonn
• base

Examples

			79 is in the sequence because it is a prime formed from the concatenation of 7 and 9, where 7 is the largest prime < 9.
8387 is in the sequence because it is a prime formed from the concatenation of 83 and 87, where 83 is the largest prime < 87.
		

Links

• K. D. Bajpai, Table of n, a(n) for n = 1..7768

Crossrefs

Cf. A000040, A084667, A084669, A151799, A151800, A280357, A280388.

Programs

Magma

[p : n in[3..200] | IsPrime (p) where p is Seqint (Intseq (n) cat Intseq (PreviousPrime (n)))];

Mathematica

Select[Table[FromDigits[Join[IntegerDigits[Prime[PrimePi[n - 1]]], IntegerDigits[n]]], {n,3,1000}], PrimeQ]

PARI

terms(n) = my(i=0, x=3); while(1, my(cc=eval(Str(precprime(x-1), x))); if(ispseudoprime(cc), print1(cc, ", "); i++); if(i==n, break); x++)
/* Print initial 40 terms as follows: */
terms(40) \\ Felix Fröhlich, Jan 05 2017