A155093 - OEIS

%I #8 May 08 2019 05:18:53

%S 13,23,61,83,103,107,151,181,277,281,283,311,313,353,383,431,587,601,

%T 631,643,647,653,683,701,761,787,821,823,827,857,877,881,883,1021,

%U 1031,1033,1061,1063

%N Primes k such that the largest digit of the concatenation of k and the k-th prime is an even nonprime.

%C The only even nonprime digits that are the last digit of a prime are 4, 6, and 8.

%e k=13 is a term because 13 is prime, prime(13)=41, and concatenation(13,41) = 1341, whose largest digit is 4, an even nonprime.

%e k=23 is a term because 23 is prime, prime(23)=83, and the largest digit of 2383 is 8, an even nonprime.

%e k=61 is a term because 61 is prime, prime(61)=283, and the largest digit of 61283 is 8.

%p A054055 := proc(n) max(op(convert(n,base,10))) ; end proc: cat2 := proc(a,b) dgs := max(1,1+ilog10(b)) ; a*10^dgs+b ; end proc: isA155093 := proc(n) local p,m ; if isprime(n) then p := ithprime(n) ; m := A054055(cat2(n,p)) ; return m <> 2 and type(m,'even') ; else false; end if; end proc: for i from 1 to 2400 do if isA155093(i) then printf("%d,",i) ; fi; od: # _R. J. Mathar_, Oct 22 2009

%Y Cf. A000027, A000040, A141468.

%K nonn,base

%O 1,1

%A _Juri-Stepan Gerasimov_, Jan 20 2009

%E Edited by _Jon E. Schoenfield_, May 07 2019