A005097 -id:A005097 - cp's OEIS frontend

A097069 Positive integers n such that 2n - 9 is prime.

6, 7, 8, 10, 11, 13, 14, 16, 19, 20, 23, 25, 26, 28, 31, 34, 35, 38, 40, 41, 44, 46, 49, 53, 55, 56, 58, 59, 61, 68, 70, 73, 74, 79, 80, 83, 86, 88, 91, 94, 95, 100, 101, 103, 104, 110, 116, 118, 119, 121, 124, 125, 130, 133, 136, 139, 140, 143, 145, 146, 151, 158, 160

Offset: 1

Douglas Winston (douglas.winston(AT)srupc.com), Sep 15 2004

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Cf. A000040, A086801.
Numbers n such that 2n+k is prime: A005097 (k=1), A067076 (k=3), A089038 (k=5), A105760 (k=7), A155722 (k=9), A101448 (k=11), A153081 (k=13), A089559 (k=15), A173059 (k=17), A153143 (k=19).
Numbers n such that 2n-k is prime: A006254 (k=1), A098090 (k=3), A089253 (k=5), A089192 (k=7), this seq(k=9), A097338 (k=11), A097363 (k=13), A097480 (k=15), A098605 (k=17), A097932 (k=19).

[n: n in [6..160] | IsPrime(2*n-9)];  // Bruno Berselli, Mar 05 2011

(Prime[Range[2,100]]+9)/2 (* Vladimir Joseph Stephan Orlovsky, Feb 08 2010 *)
Select[Range[4, 200], PrimeQ[2 # - 9] &] (* Vincenzo Librandi, Oct 16 2012 *)

is(n)=isprime(2*n-9) \\ Charles R Greathouse IV, Apr 28 2015

Half of p+9 where p is a prime greater than 2.

A097932 Positive integers n such that 2n-19 is prime.

Original entry on oeis.org

11, 12, 13, 15, 16, 18, 19, 21, 24, 25, 28, 30, 31, 33, 36, 39, 40, 43, 45, 46, 49, 51, 54, 58, 60, 61, 63, 64, 66, 73, 75, 78, 79, 84, 85, 88, 91, 93, 96, 99, 100, 105, 106, 108, 109, 115, 121, 123, 124, 126, 129, 130, 135, 138, 141, 144, 145, 148, 150, 151, 156, 163

Offset: 1

Douglas Winston (douglas.winston(AT)srupc.com), Sep 21 2004

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Cf. A000040.
Numbers n such that 2n+k is prime: A005097 (k=1), A067076 (k=3), A089038 (k=5), A105760 (k=7), A155722 (k=9), A101448 (k=11), A153081 (k=13), A089559 (k=15), A173059 (k=17), A153143 (k=19).
Numbers n such that 2n-k is prime: A006254 (k=1), A098090 (k=3), A089253 (k=5), A089192 (k=7), A097069 (k=9), A097338 (k=11), A097363 (k=13), A097480 (k=15), A098605 (k=17), this sequence (k=19).

(Prime[Range[2,100]]+19)/2 (* Vladimir Joseph Stephan Orlovsky, Feb 08 2010 *)
Select[Range[10,200],PrimeQ[2#-19]&] (* Harvey P. Dale, May 08 2017 *)

Half of p+19 where p is a prime greater than 2.

A153081 Nonnegative numbers k such that 2k + 13 is prime.

Original entry on oeis.org

0, 2, 3, 5, 8, 9, 12, 14, 15, 17, 20, 23, 24, 27, 29, 30, 33, 35, 38, 42, 44, 45, 47, 48, 50, 57, 59, 62, 63, 68, 69, 72, 75, 77, 80, 83, 84, 89, 90, 92, 93, 99, 105, 107, 108, 110, 113, 114, 119, 122, 125, 128, 129, 132, 134, 135, 140, 147, 149, 150, 152, 159, 162, 167

Offset: 1

Vincenzo Librandi, Dec 18 2008

Or, (p-13)/2 for primes p >= 13.
a(n) = (A000040(n+5) - 13)/2.
a(n) = A005097(n+4) - 6.
a(n) = A067076(n+4) - 5.
a(n) = A089038(n+3) - 4.
a(n) = A105760(n+2) - 3.
a(n) = A101448(n+1) - 1.
a(n) = A089559(n-1) + 1 for n > 1.

			For k = 7, 2*k+13 = 27 is not prime, so 7 is not in the sequence;
for k = 8, 2*k+13 = 29 is prime, so 8 is in the sequence.

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Cf. A000040 (prime numbers).
Numbers n such that 2n+k is prime: A005097 (k=1), A067076 (k=3), A089038 (k=5), A105760 (k=7), A155722 (k=9), A101448 (k=11), this seq (k=13), A089559 (k=15), A173059 (k=17), A153143 (k=19).
Numbers n such that 2n-k is prime: A006254 (k=1), A098090 (k=3), A089253 (k=5), A089192 (k=7), A097069 (k=9), A097338 (k=11), A097363 (k=13), A097480 (k=15), A098605 (k=17), A097932 (k=19).

Filtered([0..200], k-> IsPrime(2*k+13) ); # G. C. Greubel, May 22 2019

[ n: n in [0..200] | IsPrime(2*n+13) ];

(Prime[Range[6,100]]-13)/2 (* Vladimir Joseph Stephan Orlovsky, Feb 08 2010 *)
Select[Range[0,200],PrimeQ[2#+13]&] (* Harvey P. Dale, Mar 02 2015 *)

is(n)=isprime(2*n+13) \\ Charles R Greathouse IV, Jul 12 2016

[n for n in (0..200) if is_prime(2*n+13) ] # G. C. Greubel, May 22 2019

Edited and extended by Klaus Brockhaus, Dec 22 2008

Definition clarified by Zak Seidov, Jul 11 2014

A097338 Positive integers n such that 2n-11 is prime.

Original entry on oeis.org

7, 8, 9, 11, 12, 14, 15, 17, 20, 21, 24, 26, 27, 29, 32, 35, 36, 39, 41, 42, 45, 47, 50, 54, 56, 57, 59, 60, 62, 69, 71, 74, 75, 80, 81, 84, 87, 89, 92, 95, 96, 101, 102, 104, 105, 111, 117, 119, 120, 122, 125, 126, 131, 134, 137, 140, 141, 144, 146, 147, 152, 159, 161

Offset: 1

Douglas Winston (douglas.winston(AT)srupc.com), Sep 17 2004

Shawn A. Broyles, Table of n, a(n) for n = 1..1000

Cf. A000040.
Numbers n such that 2n+k is prime: A005097 (k=1), A067076 (k=3), A089038 (k=5), A105760 (k=7), A155722 (k=9), A101448 (k=11), A153081 (k=13), A089559 (k=15), A173059 (k=17), A153143 (k=19).
Numbers n such that 2n-k is prime: A006254 (k=1), A098090 (k=3), A089253 (k=5), A089192 (k=7), A097069 (k=9), this sequence (k=11), A097363 (k=13), A097480 (k=15), A098605 (k=17), A097932 (k=19).

(Prime[Range[2,100]]+11)/2 (* Vladimir Joseph Stephan Orlovsky, Feb 08 2010 *)
Select[Range[6,200],PrimeQ[2#-11]&] (* Harvey P. Dale, May 24 2014 *)

is(n)=isprime(2*n-11) \\ Charles R Greathouse IV, Jul 12 2016

Half of p+11 where p is a prime greater than 2.

A097480 Positive integers n such that 2n-15 is prime.

Original entry on oeis.org

9, 10, 11, 13, 14, 16, 17, 19, 22, 23, 26, 28, 29, 31, 34, 37, 38, 41, 43, 44, 47, 49, 52, 56, 58, 59, 61, 62, 64, 71, 73, 76, 77, 82, 83, 86, 89, 91, 94, 97, 98, 103, 104, 106, 107, 113, 119, 121, 122, 124, 127, 128, 133, 136, 139, 142, 143, 146, 148, 149, 154, 161, 163

Offset: 1

Douglas Winston (douglas.winston(AT)srupc.com), Sep 19 2004

Shawn A. Broyles, Table of n, a(n) for n = 1..1000

Cf. A000040, A098602, A098603.
Numbers n such that 2n+k is prime: A005097 (k=1), A067076 (k=3), A089038 (k=5), A105760 (k=7), A155722 (k=9), A101448 (k=11), A153081 (k=13), A089559 (k=15), A173059 (k=17), A153143 (k=19).
Numbers n such that 2n-k is prime: A006254 (k=1), A098090 (k=3), A089253 (k=5), A089192 (k=7), A097069 (k=9), A097338 (k=11), A097363 (k=13), this sequence (k=15), A098605 (k=17), A097932 (k=19).

(Prime[Range[2,100]]+15)/2 (* Vladimir Joseph Stephan Orlovsky, Feb 08 2010 *)
Select[Range[9,200],PrimeQ[2#-15]&] (* Harvey P. Dale, Apr 04 2021 *)

is(n)=isprime(2*n-15) \\ Charles R Greathouse IV, Jul 12 2016

Half of p+15 where p is a prime greater than 2.

A098605 Positive integers n such that 2n - 17 is prime.

Original entry on oeis.org

10, 11, 12, 14, 15, 17, 18, 20, 23, 24, 27, 29, 30, 32, 35, 38, 39, 42, 44, 45, 48, 50, 53, 57, 59, 60, 62, 63, 65, 72, 74, 77, 78, 83, 84, 87, 90, 92, 95, 98, 99, 104, 105, 107, 108, 114, 120, 122, 123, 125, 128, 129, 134, 137, 140, 143, 144, 147, 149, 150, 155, 162

Offset: 1

Douglas Winston (douglas.winston(AT)srupc.com), Sep 20 2004

Shawn A. Broyles, Table of n, a(n) for n = 1..1000

Cf. A000040, A098602, A098603.
Numbers n such that 2n+k is prime: A005097 (k=1), A067076 (k=3), A089038 (k=5), A105760 (k=7), A155722 (k=9), A101448 (k=11), A153081 (k=13), A089559 (k=15), A173059 (k=17), A153143 (k=19).
Numbers n such that 2n-k is prime: A006254 (k=1), A098090 (k=3), A089253 (k=5), A089192 (k=7), A097069 (k=9), A097338 (k=11), A097363 (k=13), A097480 (k=15), this sequence (k=17), A097932 (k=19).

(Prime[Range[2,100]]+17)/2 (* Vladimir Joseph Stephan Orlovsky, Feb 08 2010 *)

is(n)=isprime(2*n-17) \\ Charles R Greathouse IV, Jul 12 2016

Half of p+17 where p is a prime greater than 2.

A130290 Number of nonzero quadratic residues modulo the n-th prime.

Original entry on oeis.org

1, 1, 2, 3, 5, 6, 8, 9, 11, 14, 15, 18, 20, 21, 23, 26, 29, 30, 33, 35, 36, 39, 41, 44, 48, 50, 51, 53, 54, 56, 63, 65, 68, 69, 74, 75, 78, 81, 83, 86, 89, 90, 95, 96, 98, 99, 105, 111, 113, 114, 116, 119, 120, 125, 128, 131, 134, 135, 138, 140, 141, 146, 153, 155, 156, 158

Offset: 1

M. F. Hasler, May 21 2007

Row lengths for formatting A063987 as a table: The number of nonzero quadratic residues modulo a prime p equals floor(p/2), or (p-1)/2 if p is odd. The number of squares including 0 is (p+1)/2, if p is odd (rows prime(i) of A096008 formatted as a table). In fields of characteristic 2, all elements are squares. For any m > 0, floor(m/2) is the number of even positive integers less than or equal to m, so a(n) also equals the number of even positive integers less than or equal to the n-th prime. For all n > 0, A130290(n+1) = A005097(n) = A102781(n+1) = A102781(n+1) = A130291(n+1)-1 = A111333(n+1)-1 = A006254(n)-1.
From Vladimir Shevelev, Jun 18 2016: (Start)
a(1)+2 and, for n >= 2, a(n)+1 is the smallest k such that there exists 0 < k_1 < k with the condition k_1^2 == k^2 (mod prime(n)).
Indeed, for n >= 2, if prime(n) = 4*t+1 then k = 2*t+1 = a(n)+1, since (2*t+1)^2 == (2*t)^2 (mod prime(n)) and there cannot be a smaller value of k; if prime(n) = 4*t-1, then k = 2*t = a(n)+1, since (2*t)^2 == (2*t-1)^2 (mod prime(n)). (End)
a(n) is the number of pairs (a,b) such that a + b = prime(n) with 1 <= a <= b. - Nicholas Leonard, Oct 02 2022

			a(1)=1 since the only nonzero element of Z/2Z equals its square.
a(3)=2 since 1=1^2=(-1)^2 and 4=2^2=(-2)^2 are the only nonzero squares in Z/5Z.
a(1000000) = 7742931 = (prime(1000000)-1)/2.

Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Eric Weisstein's World of Mathematics, Quadratic Residue
Wikipedia, Quadratic Residue

Essentially the same as A005097.
Cf. A102781 (Number of even numbers less than the n-th prime), A063987 (quadratic residues modulo the n-th prime), A006254 (Numbers n such that 2n-1 is prime), A111333 (Number of odd numbers <= n-th prime), A000040 (prime numbers), A130291.
Appears in A217983. - Johannes W. Meijer, Oct 25 2012

[Floor((NthPrime(n))/2): n in [1..60]]; // Vincenzo Librandi, Jan 16 2013

A130290 := proc(n): if n =1 then 1 else (A000040(n)-1)/2 fi: end: A000040 := proc(n): ithprime(n) end: seq(A130290(n), n=1..55); # Johannes W. Meijer, Oct 25 2012

Quotient[Prime[Range[66]], 2] (* Vladimir Joseph Stephan Orlovsky, Sep 20 2008 *)

PARI
```
A130290(n) = prime(n)>>1
```

from sympy import prime
def A130290(n): return prime(n)//2  # Chai Wah Wu, Jun 04 2022

a(n) = floor( A000040(n)/2 ) = #{ even positive integers <= A000040(n) }
a(n) = A055034(A000040(n)), n>=1. - Wolfdieter Lang, Sep 20 2012
a(n) = A005097(n-[n>1]) = A005097(max(n-1,1)). - M. F. Hasler, Dec 13 2019

A173059 Nonnegative numbers k such that 2*k + 17 is prime.

Original entry on oeis.org

0, 1, 3, 6, 7, 10, 12, 13, 15, 18, 21, 22, 25, 27, 28, 31, 33, 36, 40, 42, 43, 45, 46, 48, 55, 57, 60, 61, 66, 67, 70, 73, 75, 78, 81, 82, 87, 88, 90, 91, 97, 103, 105, 106, 108, 111, 112, 117, 120, 123, 126, 127, 130, 132, 133, 138, 145, 147, 148, 150, 157, 160, 165

Offset: 1

Vladimir Joseph Stephan Orlovsky, Feb 08 2010

Shawn A. Broyles, Table of n, a(n) for n = 1..1000

Numbers n such that 2n+k is prime: A005097 (k=1), A067076 (k=3), A089038 (k=5), A105760 (k=7), A155722 (k=9), A101448 (k=11), A153081 (k=13), A089559 (k=15), this seq (k=17), A153143 (k=19).

Numbers n such that 2n-k is prime: A006254 (k=1), A098090 (k=3), A089253 (k=5), A089192 (k=7), A097069 (k=9), A097338 (k=11), A097363 (k=13), A097480 (k=15), A098605 (k=17), A097932 (k=19).

Filtered([0..200], k-> IsPrime(2*k+17) ); # G. C. Greubel, May 22 2019

[n: n in [0..200] | IsPrime(2*n+17) ]; // G. C. Greubel, May 22 2019

Mathematica
```
(Prime[Range[7,100]]-17)/2
```

is(n)=isprime(2*n+17) \\ Charles R Greathouse IV, Feb 17 2017

[n for n in (0..200) if is_prime(2*n+17) ] # G. C. Greubel, May 22 2019

Definition clarified by Zak Seidov, Jul 11 2014

A016014 Least k such that 2nk + 1 is a prime.

Original entry on oeis.org

1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 3, 2, 1, 1, 3, 3, 1, 5, 1, 1, 2, 1, 2, 2, 1, 2, 2, 1, 1, 5, 3, 1, 2, 1, 1, 2, 3, 1, 3, 1, 4, 2, 1, 2, 3, 3, 1, 2, 1, 1, 3, 1, 1, 3, 1, 2, 2, 6, 2, 3, 3, 1, 2, 1, 3, 2, 1, 1, 2, 4, 3, 2, 1, 1, 3, 3, 1, 2, 4, 1, 5, 1, 2, 6, 1, 2, 2, 1, 1, 3, 7, 2, 5, 1, 1, 2, 1, 1

Offset: 1

Robert G. Wilson v

nonn

Is the sequence bounded? - Zak Seidov, Mar 25 2014
Answer: No, for any given N a number n such that a(n) > N can be constructed by the Chinese Remainder Theorem, see A239727. - Charles R Greathouse IV, Mar 25 2014
a(n) = 1 for n in A005097. - Robert Israel, Oct 26 2016

Zak Seidov, Table of n, a(n) for n = 1..10000

Cf. A005097, A103961.
A070846 contains the corresponding primes.
Records are in A239746 with indices in A239727.

f:= proc(n) local k;
     for k from 1 do if isprime(2*n*k+1) then return k fi od
end proc:
map(f, [$1..100]); # Robert Israel, Oct 26 2016

Do[k = 1; cp = n*k + 1; While[ ! PrimeQ[cp], k++; cp = n*k + 1]; Print[k], {n, 2, 400, 2}] (* Lei Zhou, Feb 23 2005 *)
lk[n_]:=Module[{k=1},While[!PrimeQ[2n k+1],k++];k]; Array[lk,100] (* Harvey P. Dale, Apr 23 2023 *)

a(n)=my(k); while(!isprime(2*n*(k++)+1),);k \\ Charles R Greathouse IV, Mar 25 2014

from sympy import isprime
def a(n):
    k = 1
    while not isprime(2*n*k + 1): k += 1
    return k
print([a(n) for n in range(1, 100)]) # Michael S. Branicky, Mar 28 2022

A037225 a(n) = phi(2n+1).

Original entry on oeis.org

1, 2, 4, 6, 6, 10, 12, 8, 16, 18, 12, 22, 20, 18, 28, 30, 20, 24, 36, 24, 40, 42, 24, 46, 42, 32, 52, 40, 36, 58, 60, 36, 48, 66, 44, 70, 72, 40, 60, 78, 54, 82, 64, 56, 88, 72, 60, 72, 96, 60, 100, 102, 48, 106, 108, 72, 112, 88, 72, 96, 110, 80, 100, 126, 84, 130

Offset: 0

N. J. A. Sloane

Bisection of A000010 (cf. A062570).
From Alain Rocchelli, Jun 28 2023: (Start)
If 2*n+1 has r distinct odd prime factors, 2^r divides a(n).
Conjectures:
1) For any composite integer 2*n+1, a(n) doesn't divide 2*n.
2) For all n, a(n) is never equal to n. (End)

T. D. Noe, Table of n, a(n) for n = 0..10000
John Brillhart, J. S. Lomont, and Patrick Morton, Cyclotomic properties of the Rudin-Shapiro polynomials, J. Reine Angew. Math. 288 (1976), 37-65; see Table 2; MR0498479 (58 #16589). - From _N. J. A. Sloane_, Jun 06 2012
V. I. Levenshtein, Conflict-avoiding codes and cyclic triple systems, Problemy Peredachi Informatsii, 43 (No. 3, 2007), 39-53 (in Russian).
V. I. Levenshtein, Conflict-avoiding codes and cyclic triple systems, Problems of Information. Transmission, 43 (2007), 199-212 (translation from Russian).
N. J. A. Sloane, Families of Essentially Identical Sequences, Mar 24 2021 (Includes this sequence).
Wikipedia, Lehmer's totient problem.

Cf. A000010, A062570, A072451, A217739.

[EulerPhi(2*n+1): n in [0..70]]; // Vincenzo Librandi, Jul 16 2019

Table[EulerPhi[2 n + 1], {n, 0, 80}] (* Vincenzo Librandi, Jul 16 2019 *)

a(n) = eulerphi(2*n+1) \\ Amiram Eldar, Nov 17 2022

Sum_{k=0..n} a(k) ~ c * n^2, where c = 8/Pi^2 = 0.810569... (A217739). - Amiram Eldar, Nov 17 2022
a(n) = 2*n iff 2*n+1 is prime, see A005097. - Alain Rocchelli, Jun 22 2023
From Peter Bala, Feb 01 2024: (Start)
Odd bisection of A000010.
a(n) = 2*A072451(n) for n >= 1.
G.f.: Sum_{n >= 1} phi(2*n+1)*x^(2*n+1) = Sum_{n >= 1} moebius(n)*x^(2*n-1)*(1 + x^(4*n-2))/(1 - x^(4*n-2))^2 = x + 2*x^3 + 4*x^5 + 6*x^7 + 6*x^9 + .... (End)

cp's OEIS Frontend

A097069 Positive integers n such that 2n - 9 is prime.

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A097932 Positive integers n such that 2n-19 is prime.

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A153081 Nonnegative numbers k such that 2k + 13 is prime.

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A097338 Positive integers n such that 2n-11 is prime.

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A097480 Positive integers n such that 2n-15 is prime.

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A098605 Positive integers n such that 2n - 17 is prime.

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A130290 Number of nonzero quadratic residues modulo the n-th prime.

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A016014 Least k such that 2nk + 1 is a prime.