A280388 Primes formed from the concatenation of n and previousprime(n).

43, 53, 97, 107, 1613, 2423, 3331, 3631, 4241, 4643, 5147, 5347, 5653, 5953, 6361, 6661, 6761, 6967, 7573, 7673, 7873, 8179, 8783, 9689, 102101, 106103, 108107, 111109, 114113, 116113, 123113, 125113, 129127, 130127, 135131, 137131, 144139, 145139, 147139, 148139

Offset: 1

Author

K. D. Bajpai, Jan 01 2017

Keywords

• nonn
• base

Examples

			43 is in the sequence because it is prime formed from the concatenation of 4 and 3, where 3 is largest prime < 4.
1613 is in the sequence because it is prime formed from the concatenation of 16 and 13, where 13 is largest prime < 16.
		

Links

• K. D. Bajpai, Table of n, a(n) for n = 1..9135

Crossrefs

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

Programs

Maple

with(numtheory): select( isprime,[seq((n*10^floor(evalf(log10(prevprime(n))+1, 100))+prevprime(n)), n=3..500)]);
P:=proc(i) local a, n, c; c:=1; for n from 3 by 1 to i  do  a:=n*10^floor(evalf(log10(prevprime(n))+1, 100))+prevprime(n);if (isprime(a)) then lprint(c,a);c:=c+1;  fi;od; end: P(10000);

Mathematica

Select[Table[FromDigits[Join[IntegerDigits[n],IntegerDigits[NextPrime[n,-1]]]],{n,200}],PrimeQ] (* Harvey P. Dale, Oct 22 2022 *)