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 A065686 Number of primes <= prime(n) which begin with a 7. 2
 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 (list; graph; refs; listen; history; text; internal format)
 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: A030241 A062750 A193669 * A158411 A065680 A093391 Adjacent sequences:  A065683 A065684 A065685 * A065687 A065688 A065689 KEYWORD base,nonn AUTHOR Reinhard Zumkeller, Nov 13 2001 STATUS approved

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Last modified June 17 17:05 EDT 2021. Contains 345085 sequences. (Running on oeis4.)