cp's OEIS Frontend

A354839 Beginning with 0, smallest positive integer not yet in the sequence such that the concatenation of two digits of the sequence separated by a comma is prime.

%I A354839 #20 Jul 07 2022 02:28:25
%S A354839 0,2,3,1,7,9,70,5,30,20,21,10,22,31,11,12,32,33,13,14,15,34,16,17,18,
%T A354839 35,36,19,71,37,38,39,72,90,23,73,74,75,91,76,77,92,93,78,94,79,700,
%U A354839 24,100,25,95,96,101,97,98,99,701,102,300,26,103,104,105,301
%N A354839 Beginning with 0, smallest positive integer not yet in the sequence such that the concatenation of two digits of the sequence separated by a comma is prime.
%e A354839 a(4)=1 because this is the first number not in the sequence whose first digit is 3 (last digit of a(3)), concatenated with its first digit 1, is prime: 31.
%e A354839 a(14)=31 because this is the first number not in the sequence whose first digit is 2 (last digit of a(13)), concatenated with its first digit 3, is prime: 23.
%o A354839 (Python)
%o A354839 from sympy import isprime
%o A354839 from itertools import count, islice
%o A354839 def agen(): # generator of terms
%o A354839     aset, k, mink = {0}, 0, 1; yield 0
%o A354839     for n in count(2):
%o A354839         k, prevdig = mink, str(k%10)
%o A354839         while k in aset or not isprime(int(prevdig+str(k)[0])): k += 1
%o A354839         aset.add(k); yield k
%o A354839         while mink in aset: mink += 1
%o A354839 print(list(islice(agen(), 64))) # _Michael S. Branicky_, Jun 09 2022
%Y A354839 Cf. A074721, A158652, A152604-A152609, A152136, A152607.
%K A354839 nonn,easy,base
%O A354839 0,2
%A A354839 _Carole Dubois_, Jun 08 2022