A072348 - OEIS

%I #7 Sep 24 2024 22:25:28

%S 15,13,13,15,14,16,15,15,13,14,11,10,12,9,5,12,10,10,8,8,12,13,13,13,

%T 9,11,8,12,12,13,11,10,11,10,11,9,9,11,10,12,5,11,11,12,12,11,12,10,

%U 11,9,7,10,14,11,14,11,9,10,10,11,11,8,12,10,12,12,11,13,11,15,11,11,10,12

%N Number of primes whose decimal representation has the form rns where r and s are single digits.

%C a(2437) = 0 and a(n)>0 for n<2437, see A069691(2437) = 0;

%C a(1660) = 1 and a(n)>1 for n<1660, A069691(1660) = 216607;

%C a(n) <= 16 for all n, a(5) = 16, conjecture: a(n)<16 for n>5.

%e The 16 prime sandwiches for n=5: 151, 157, 251, 257, 353, 359, 457, 557, 653, 659, 751, 757, 853, 857, 859 and 953.

%K nonn,base

%O 0,1

%A _Reinhard Zumkeller_, Jul 17 2002

%E Offset corrected by _Sean A. Irvine_, Sep 24 2024