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.

Showing 1-3 of 3 results.

A090519 Smallest prime p such that floor((10^n)/p) is prime, or 0 if no such number exists.

Original entry on oeis.org

2, 13, 23, 13, 89, 19, 7, 47, 67, 13, 17, 157, 17, 313, 107, 409, 151, 773, 149, 409, 109, 13, 29, 211, 7, 19, 149, 431, 859, 43, 109, 167, 277, 13, 2293, 173, 907, 107, 1087, 617, 449, 1013, 73, 1249, 743, 109, 233, 499, 191, 479
Offset: 1

Views

Author

Amarnath Murthy, Dec 07 2003

Keywords

Comments

Conjecture: No term is zero. Subsidiary Sequence: Number of primes in floor((10^n)/p), p is a prime. a(1) = 3, the primes are 10/2, floor(10/3) and 10/5.

Examples

			a(5) = 89, as floor((10^5)/89) = 1123 is the largest such prime.

Links

Crossrefs

Cf. A090517, A090518, A090520.

Programs

  • Maple
    f:= proc(n) local t,p;
t:= 10^n;
p:= 1;
while p < t/2 do
  p:= nextprime(p);
  if isprime(floor(t/p)) then return p fi
od;
0
end proc:
map(f, [$1..50]); # Robert Israel, Jul 30 2023
  • Mathematica
    <Ryan Propper, Jun 19 2005 *)

Extensions

Corrected and extended by Ryan Propper, Jun 19 2005

A090520 Primes arising in A090519, or 0 if A090519(n)= 0.

Original entry on oeis.org

5, 7, 43, 769, 1123, 52631, 1428571, 2127659, 14925373, 769230769, 5882352941, 6369426751, 588235294117, 319488817891, 9345794392523, 24449877750611, 662251655629139, 1293661060802069, 67114093959731543, 244498777506112469
Offset: 1

Views

Author

Amarnath Murthy, Dec 07 2003

Keywords

Comments

Conjecture: No term is zero.

Crossrefs

Cf. A090517, A090518, A090519.

Programs

  • PARI
    A090520(n)={ local(p,tenn) ; p=2 ; tenn=10^n ; while(tenn/p>=2, if( isprime(floor(tenn/p)), return(floor(tenn/p)) ) ; p=nextprime(p+1) ; ) ; return(0) ; } { for(n=1,40, print1(A090520(n),",") ; ) } - R. J. Mathar, Nov 19 2006

Extensions

Corrected and extended by R. J. Mathar, Nov 19 2006

A090517 Least k such that floor[(10^n)/k] is prime.

Original entry on oeis.org

2, 9, 12, 13, 28, 19, 7, 27, 12, 13, 17, 74, 17, 14, 82, 66, 36, 44, 9, 36, 21, 13, 9, 90, 7, 19, 149, 51, 321, 35, 12, 14, 140, 13, 28, 42, 34, 36, 153, 155, 133, 46, 73, 106, 162, 109, 122, 42, 62, 422, 29, 231, 38, 34, 340, 295, 151, 197, 94, 19, 17, 83, 131, 66, 36
Offset: 1

Views

Author

Amarnath Murthy, Dec 07 2003

Keywords

Examples

			a(1) = 2 as 10/2 = 5 is a prime. a(3) = 12 as floor[1000/12] = 83 is prime.

Crossrefs

Cf. A090518, A090519, A090520.

Programs

  • Mathematica
    lkp[n_]:=Module[{k=2,c=10^n},While[!PrimeQ[Floor[c/k]],k++];k]; Array[ lkp,70] (* Harvey P. Dale, Jul 09 2018 *)

Extensions

Corrected and extended by Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Jan 28 2004
Further terms from David Wasserman, Dec 20 2005
Showing 1-3 of 3 results.

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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