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Primes formed from the concatenation of previousprime(n) and n.
1

%I #10 Jan 09 2017 11:26:46

%S 23,79,3137,3739,4751,6163,8387,8389,109111,113117,113123,151153,

%T 151157,157163,167173,173177,181183,199207,199211,211213,211217,

%U 211219,233239,241249,251257,257263,263267,263269,271273,271277,277279,283289,317321,317323,317327

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

%H K. D. Bajpai, <a href="/A280576/b280576.txt">Table of n, a(n) for n = 1..7768</a>

%e 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.

%e 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.

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

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

%o (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++)

%o /* Print initial 40 terms as follows: */

%o terms(40) \\ _Felix Fröhlich_, Jan 05 2017

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

%K nonn,base

%O 1,1

%A _K. D. Bajpai_, Jan 05 2017