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A065681
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Number of primes <= prime(n) which begin with a 2.
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2
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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, 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, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19
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OFFSET
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1,9
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LINKS
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EXAMPLE
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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).
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MATHEMATICA
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Accumulate@ Array[Boole[First@ IntegerDigits@ Prime@ # == 2] &, 87] (* Michael De Vlieger, Jun 14 2018 *)
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PROG
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(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
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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STATUS
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approved
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