cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A248858 a(n) is number of digits of the smallest term of the sequence A248857 which is of the form 4^n*(5^(2n-1)*10^m-1).

Original entry on oeis.org

4, 5, 7, 10, 14, 14, 16, 17, 42, 24, 47, 25, 29, 39, 31, 40, 41, 45, 40, 69, 46, 65, 807, 128, 67, 89, 55, 217, 87, 76, 74, 72, 99, 70, 75, 144, 78, 213, 96, 233, 114, 103, 457, 108, 156, 163, 140, 97, 127, 270, 109, 127, 167, 135, 128, 131, 196, 133, 309, 138, 148, 140, 813, 169, 135, 148, 235, 7235
Offset: 1

Views

Author

Farideh Firoozbakht, Jan 06 2015

Keywords

Examples

			a(1) = number of digits of 1996 = 4.
a(2) = number of digits of 19984 = 5.
a(3) = number of digits of 1999936 = 7 and
a(68)=7235, because smallest prime of the form 5^(2*68-1)*10^m -1 is 5^135*10^7099-1 and 4^68(5^135*10^7099-1) has 7235 digits.

Crossrefs

Cf. A248857.

A248854 Numbers n such that n - pi(n) is a power of 10.

Original entry on oeis.org

1, 2, 3, 16, 17, 132, 1196, 11373, 110486, 1084604, 10708554, 106091744, 1053422338, 10475688326, 104287176418, 1039019056245, 10358018863852, 103307491450819, 1030734020030317, 10287026204717357, 102692313540015923, 1025351434864118025, 10239531292310798955, 102270102190290407385
Offset: 1

Views

Author

Farideh Firoozbakht, Dec 31 2014

Keywords

Comments

For n > 1 there exist at most two n-digit terms. If n is a term of the sequence and n + 1 is prime then n + 1 is also in the sequence.
Numbers n such that pi(n) is equal to n - 10^floor(log(10, n)). - Farideh Firoozbakht, Jan 01 2015

Crossrefs

Cf. A000720, A248856, A248857.

Programs

  • Maple
    A[1]:= 1: A[2]:= 2: A[3]:= 3:
count:= 3:
for k from 1 to 8 do
  x:= 10^k; y:= x + numtheory:-pi(x);
  while x < y and y < 10^(k+1) do
    x:= y; y:= 10^k + numtheory:-pi(x);
  od;
  count:= count+1; A[count]:= x;
  if isprime(x+1) then
      count:= count+1; A[count]:= x+1
  fi;
od:
seq(A[i],i=1..count); # Robert Israel, Dec 31 2014
  • Mathematica
    Select[Range[1000], IntegerQ[Log[10, # - PrimePi[#]]] &] (* Alonso del Arte, Jan 01 2015 *)
  • PARI
    for(n=1,10^3,s=digits(n-primepi(n)-1);if(s==[]||vecmin(s)==9,print1(n,", "))) \\ Derek Orr, Jan 02 2015

Extensions

a(19)-a(24) from Giovanni Resta, Jun 07 2020
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Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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