A065686 Number of primes <= prime(n) which begin with a 7.

0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4

Offset

1,20

Links

Harry J. Smith, Table of n, a(n) for n = 1..1000

Example

a(3) = 0, a(4) = 1. a(664579) = 71564 (A000040(664579) = 9999991 is the largest prime < 10^7).

Prog

(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) == 7, a++); p=nextprime(p+1)); write("b065686.txt", n, " ", a) ) } \\ Harry J. Smith, Oct 26 2009

Crossrefs

Cf. A065680, A000040.

Sequence in context: A062750 A368420 A193669 * A158411 A065680 A093391

Adjacent sequences: A065683 A065684 A065685 * A065687 A065688 A065689

Keyword

base,nonn

Author

Reinhard Zumkeller, Nov 13 2001

Status

approved