reviewed

approved

proposed

reviewed

editing

proposed

Accumulate@ Array[Boole[First@ IntegerDigits@ Prime@ # == 5] &, 103] (* Michael De Vlieger, Jun 14 2018 *)

proposed

editing

editing

proposed

Harry J. Smith, <a href="/A065684/b065684.txt">Table of n, a(n) for n = 1,...,1000</a>

a(i) = 1 for 2 < i < 16 and a(16) = 2 as 53 = A000040(16) is the second prime beginning with 5. a(664579) = 72951 (A000040(664579) = 9999991 is the largest prime < 10,000,000^7).

(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) == 5, a++); p=nextprime(p+1)); write("b065684.txt", n, " ", a) ) } [From _\\ _Harry J. Smith_, Oct 26 2009]

Cf. A065680, A000040.

approved

editing

_Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), _, Nov 13 2001

(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) == 5, a++); p=nextprime(p+1)); write("b065684.txt", n, " ", a) ) } [From _Harry J. Smith (hjsmithh(AT)sbcglobal.net), _, Oct 26 2009]

Harry J. Smith, <a href="/A065684/b065684.txt">Table of n, a(n) for n=1,...,1000</a>

base,nonn,new

Harry J. Smith, <a href="b065684.txt">Table of n, a(n) for n=1,...,1000</a>

(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) == 5, a++); p=nextprime(p+1)); write("b065684.txt", n, " ", a) ) } [From Harry J. Smith (hjsmithh(AT)sbcglobal.net), Oct 26 2009]

base,nonn,new