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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A117752 Number of prime 3-digit palindromes in base n.

Original entry on oeis.org

2, 2, 3, 5, 6, 6, 13, 13, 15, 21, 20, 26, 25, 30, 36, 30, 37, 44, 50, 42, 59, 60, 61, 71, 75, 68, 89, 93, 109, 100, 111, 113, 118, 120, 133, 134, 142, 155, 162, 154, 166, 168, 187, 194, 220, 195, 207, 212, 235, 210, 243, 263, 247, 257, 293, 286, 296, 320, 304, 328
Offset: 2

Views

Author

Keywords

Comments

Is there a proof that a(n) > 0?

Crossrefs

Cf. A060873.

Programs

  • Maple
    f:= proc(n) nops(select(isprime, [seq(seq(i*(n^2+1)+j*n,j=0..n-1),i=1..n-1)])) end proc:
    map(f, [$2..100]); # Robert Israel, May 04 2020
  • PARI
    a(n)=local(t,i,j);t=0;for(i=1,n-1,for(j=0,n-1,if(isprime(i*(n^2+1)+j*n),t++)));t

A100563 Number of bases less than sqrt(n) in which n is a palindrome.

Original entry on oeis.org

0, 0, 0, 0, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 1, 2, 0, 0, 1, 2, 0, 1, 0, 1, 2, 1, 1, 1, 0, 2, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 0, 1, 2, 0, 0, 1, 1, 2, 2, 0, 0, 2, 1, 2, 0, 1, 0, 1, 1, 2, 1, 3, 0, 2, 1, 0, 0, 1, 1, 2, 1, 0, 0, 0, 2, 0, 2, 1, 2, 1, 0, 3, 1, 0, 1, 1, 0, 2, 2, 2, 0, 0, 0, 1, 2, 1, 3, 1, 0, 0, 2, 2
Offset: 1

Views

Author

Gordon Hamilton, Nov 29 2004

Keywords

Comments

Is there a number m such that a(n) > 0 for all n > m? I call the set of numbers for which a(n)=0 "unkempt" for refusing to use a mirror in any base. Is there an infinite number of unkempt numbers? a(n) can be arbitrarily large.
The sequence A123586 gives the values of n where a(n)=0. - Robert G. Wilson v, Nov 01 2014
Is there a closed-form formula for this function? - Robert G. Wilson v, Nov 01 2014
From Robert G. Wilson v, Nov 26 2014: (Start)
The first occurrence, beginning at 0, of n is: 1, 5, 17, 65, 121, 562, 1432, 1477, 4369, 36582, 35101, 86677, 83161, 360361, 291721, 720721, 887041, 1496881, 1670761, 3931201, 3341521, 5654881, 7207201, 7761601,...
Positions where a(n)=k:
k = 0: A123586;
k = 1: 5, 7, 9, 10, 13, 15, 16, 20, 23, 25, 27, 28, 29, 33, 34, 36, 37, 38, 40, ...;
k = 2: 17, 21, 26, 31, 46, 51, 52, 55, 57, 63, 67, 73, 78, 80, 82, 91, 92, 93, 98, ...;
k = 3: 65, 85, 100, 130, 154, 164, 170, 178, 191, 195, 203, 209, 242, 282, 292, ...;
k = 4: 121, 235, 255, 257, 273, 300, 325, 341, 343, 373, 400, 495, 601, 610, 626, 666, ...;
k = 5: 562, 676, 771, 819, 1009, 1111, 1220, 1333, 1365, 1441, 1543, 1978, 1981, 2000, ...;
k = 6: 1432, 2380, 2666, 2925, 3280, 4035, 4095, 4161, 4225, 4401, 4525, 4561, 4681, ...;
k = 7: 1477, 4097, 4591, 7141, 7993, 8191, 9640, 10081, 10297, 10626, 10858, 11761, ...; etc.
(End)

Examples

			100 is a palindrome in bases 3, 7 and 9, so a(100) = 3.
		

Crossrefs

Programs

  • Mathematica
    f[n_] := Module[{p}, Table[ p = IntegerDigits[n, b]; If[p == Reverse@ p, {b, p}, Sequence @@ {}], {b, 2, Sqrt@ n}]]; Array[ Length@ f@# &, 105] (* Robert G. Wilson v, Nov 01 2014 *)
  • PARI
    a(n) = {my(nb = 0); for (b=2, sqrt(n), d = digits(n, b); nb+= (Vecrev(d) == d);); nb;} \\ Michel Marcus, Nov 05 2014

Formula

a(n) = A135551(n) - A033831(n). - Robert G. Wilson v, Nov 01 2014

Extensions

a(58) from Robert G. Wilson v, Nov 05 2014
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