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.

A138887 Numbers that are not Sophie Germain primes.

Original entry on oeis.org

0, 1, 4, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 27, 28, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68
Offset: 0

Views

Author

Omar E. Pol, Apr 05 2008

Keywords

Comments

Nonnegative integers that are not in A005384.
A156660(a(n)) = 0; A053176 is a subsequence. [From Reinhard Zumkeller, Feb 18 2009]

Crossrefs

Cf. A001690, A005384, A053176, A062289, A138888, A138889, A138890, A138891.

A274915 Powers of odd non-Fermat primes.

Original entry on oeis.org

1, 7, 11, 13, 19, 23, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 121, 127, 131, 137, 139, 149, 151, 157, 163, 167, 169, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 263, 269, 271, 277, 281, 283, 293, 307, 311
Offset: 1

Views

Author

Juri-Stepan Gerasimov, Nov 11 2016

Keywords

Comments

n is in the sequence if n = p^m where p is in A138889 and m >= 0. - Robert Israel, Sep 15 2017
The difference between two divisors of n is never a power of 2. The first number with this property that is not in the sequence is 91. - Robert Israel, Sep 15 2017
Subsequence of A061345.

Examples

			49 is in this sequence because 49 = 7^2 and 7 is not a Fermat prime.

Links

Crossrefs

Cf. A019434, A061345, A092506, A138889, A277994.

Programs

  • Maple
    N:= 500: # to get all terms <= N
P:= select(isprime, {seq(i,i=7..N,2)}) minus {seq(2^i+1, i=1..ilog2(N))}:
sort(convert(map(p -> seq(p^k,k=0..floor(log[p](N))), P), list)); # Robert Israel, Sep 15 2017

Formula

A277994(a(n)) = 0.

Extensions

Edited, new name, and corrected by Robert Israel, Sep 15 2017

A372781 Odd numbers k such that A001221(k) < A001221(A003958(k)).

Original entry on oeis.org

7, 11, 13, 19, 23, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 77, 79, 83, 89, 93, 97, 101, 103, 107, 109, 113, 121, 127, 129, 131, 137, 139, 143, 149, 151, 155, 157, 161, 163, 167, 169, 173, 179, 181, 183, 191, 193, 197, 199, 201, 203, 209, 211, 213, 215, 217, 223
Offset: 1

Views

Author

Mike Jones, Jul 04 2024

Keywords

Examples

			31 is in the sequence because 31 = 31^1, so omega(31) = 1, but (31 - 1)^1 = 30^1 = 2^1 * 3^1 * 5^1, so omega(30) = 3, and 1 < 3.

Crossrefs

Cf. A001221, A003958, A005408, A138889.

Programs

  • Maple
    q:= n-> (f-> n::odd and f(n) nops(ifactors(k)[2])):
select(q, [$1..333])[];  # Alois P. Heinz, Jul 04 2024
  • Mathematica
    f[p_, e_] := (p - 1)^e; s[n_] := Times @@ f @@@ FactorInteger[n]; Select[Range[3, 1000, 2], PrimeNu[#] < PrimeNu[s[#]] &] (* Amiram Eldar, Jul 04 2024 *)
Showing 1-3 of 3 results.

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

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