A118956 -id:A118956 - cp's OEIS frontend

A118954 Numbers that cannot be written as 2^k + prime.

1, 2, 16, 22, 26, 28, 36, 40, 46, 50, 52, 56, 58, 64, 70, 76, 78, 82, 86, 88, 92, 94, 96, 100, 106, 112, 116, 118, 120, 122, 124, 126, 127, 134, 136, 142, 144, 146, 148, 149, 154, 156, 160, 162, 166, 170, 172, 176, 178, 184, 186, 188, 190, 196, 202, 204, 206, 208

Offset: 1

Reinhard Zumkeller, May 07 2006

nonn

A109925(a(n)) = 0.

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
Roger Crocker, A theorem concerning prime numbers, Mathematics Magazine 34:6 (1961), pp. 316+344.
P. Erdős, On integers of the form 2^k + p and some related problems, Summa Brasil. Math. 2 (1950), 113-123.
N. P. Romanoff, Über einige Sätze der additiven Zahlentheorie, Math. Ann. 57 (1934), pp. 668-678.
J. G. van der Corput, On de Polignac’s conjecture, Simon Stevin 27 (1950), pp. 99-105. Cited in MR 35298.

Complement of A118955. Subsequence of A118956. Supersequence of A006285.

Cf. A156695, A010051, A000079.

a118954 n = a118954_list !! (n-1)
a118954_list = filter f [1..] where
   f x = all (== 0) $ map (a010051 . (x -)) $ takeWhile (< x) a000079_list
-- Reinhard Zumkeller, Jan 03 2014

lst:=[]; for n in [1..208] do k:=-1; repeat k+:=1; a:=n-2^k; until a lt 1 or IsPrime(a); if a lt 1 then Append(~lst, n); end if; end for; lst; // Arkadiusz Wesolowski, Sep 02 2016

is(n)=my(k=1);while(kCharles R Greathouse IV, Sep 01 2015

n < a(n) < kn for some k < 2 and all large enough n, see Romanoff and either Erdős or van der Corput. - Charles R Greathouse IV, Sep 01 2015

A118957 Numbers of the form 2^k + p, where p is a prime less than 2^k.

Original entry on oeis.org

6, 7, 10, 11, 13, 15, 18, 19, 21, 23, 27, 29, 34, 35, 37, 39, 43, 45, 49, 51, 55, 61, 63, 66, 67, 69, 71, 75, 77, 81, 83, 87, 93, 95, 101, 105, 107, 111, 117, 123, 125, 130, 131, 133, 135, 139, 141, 145, 147, 151, 157, 159, 165, 169, 171, 175, 181, 187, 189, 195, 199

Offset: 1

Reinhard Zumkeller, May 07 2006

nonn

Complement of A118956; subsequence of A118955.

Cf. A118952, A118958, A156695.

isA118957 := proc(n)
    local twok,p ;
    twok := 1 ;
    while twok < n-1 do
        p := n-twok ;
        if isprime(p) and p < twok then
            return true;
        end if;
        twok := twok*2 ;
    end do:
    return false;
end proc:
for n from 1 to 200 do
    if isA118957(n) then
        printf("%d,",n) ;
    end if;
end do: # R. J. Mathar, Feb 27 2015

okQ[n_] := Module[{k, p}, For[k = Ceiling[Log[2, n]], k>1, k--, p = n-2^k; If[2 <= p < 2^k && PrimeQ[p], Return[True]]]; False]; Select[Range[200], okQ] (* Jean-François Alcover, Mar 11 2019 *)

is(n)=isprime(n-2^logint(n,2)) \\ Charles R Greathouse IV, Sep 01 2015; edited Jan 24 2024

from sympy import primepi
def A118957(n):
    def bisection(f,kmin=0,kmax=1):
        while f(kmax) > kmax: kmax <<= 1
        kmin = kmax >> 1
        while kmax-kmin > 1:
            kmid = kmax+kmin>>1
            if f(kmid) <= kmid:
                kmax = kmid
            else:
                kmin = kmid
        return kmax
    def f(x): return n+x-sum(primepi(min(x-(m:=1<Chai Wah Wu, Feb 23 2025

A118952(a(n)) = 1.

A118952 Characteristic function of numbers that can be written as p+2^k, where p is prime and p less than 2^k (A118957).

Original entry on oeis.org

0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1

Offset: 1

Reinhard Zumkeller, May 07 2006

nonn

0 <= a(n) <= 1, a(n) <= A109925(n);
a(A118956(n)) = 0, a(A118957(n)) = 1;
A118953(n) = a(A000040(n)).

Index entries for characteristic functions

cp's OEIS Frontend

A118954 Numbers that cannot be written as 2^k + prime.

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A118957 Numbers of the form 2^k + p, where p is a prime less than 2^k.

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A118952 Characteristic function of numbers that can be written as p+2^k, where p is prime and p less than 2^k (A118957).

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Keywords

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