%I #12 Jun 15 2018 08:09:33
%S 1,1,1,1,1,1,1,1,2,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,
%T 3,3,3,3,3,3,3,3,3,3,3,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,19,
%U 19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19,19
%N Number of primes <= prime(n) which begin with a 2.
%H Harry J. Smith, <a href="/A065681/b065681.txt">Table of n, a(n) for n = 1..1000</a>
%e After 2 and 23, 29 is the third prime beginning with 2: A000040(10) = 29, therefore a(10) = 3. a(664579) = 77025 (A000040(664579) = 9999991 is the largest prime < 10^7).
%t Accumulate@ Array[Boole[First@ IntegerDigits@ Prime@ # == 2] &, 87] (* _Michael De Vlieger_, Jun 14 2018 *)
%o (PARI) digitsIn(x)= { local(d); if (x==0, return(1)); d=1 + log(x)\log(10); if (10^d == x, d++, if (10^(d-1) > x, d--)); return(d) } MSD(x)= { return(x\10^(digitsIn(x)-1)) } { a=0; p=2; for (n=1, 1000, q=prime(n); while (p <= q, if(MSD(p) == 2, a++); p=nextprime(p+1)); write("b065681.txt", n, " ", a) ) } \\ _Harry J. Smith_, Oct 26 2009
%Y Cf. A065680, A000040.
%K base,nonn
%O 1,9
%A _Reinhard Zumkeller_, Nov 13 2001