A015921 -id:A015921 - cp's OEIS frontend

A015910 a(n) = 2^n mod n.

0, 0, 2, 0, 2, 4, 2, 0, 8, 4, 2, 4, 2, 4, 8, 0, 2, 10, 2, 16, 8, 4, 2, 16, 7, 4, 26, 16, 2, 4, 2, 0, 8, 4, 18, 28, 2, 4, 8, 16, 2, 22, 2, 16, 17, 4, 2, 16, 30, 24, 8, 16, 2, 28, 43, 32, 8, 4, 2, 16, 2, 4, 8, 0, 32, 64, 2, 16, 8, 44, 2, 64, 2, 4, 68, 16, 18, 64, 2, 16, 80, 4, 2, 64

Offset: 1

Robert G. Wilson v

nonn

2^n == 2 mod n if and only if n is a prime or a member of A001567 or of A006935. [Guy]. - N. J. A. Sloane, Mar 22 2012; corrected by Thomas Ordowski, Mar 26 2016
Known solutions to 2^n == 3 (mod n) are given in A050259.
This sequence is conjectured to include every integer k >= 0 except k = 1. A036236 includes a proof that k = 1 is not in this sequence, and n = A036236(k) solves a(n) = k for all other 0 <= k <= 1000. - David W. Wilson, Oct 11 2011
It could be argued that a(0) := 1 would make sense, e.g., thinking of "mod n" as "in Z/nZ", and/or because (anything)^0 = 1. See also A112987. - M. F. Hasler, Nov 09 2018

			a(7) = 2 because 2^7 = 128 = 2 mod 7.
a(8) = 0 because 2^8 = 256 = 0 mod 8.
a(9) = 8 because 2^9 = 512 = 8 mod 9.

Richard K. Guy, Unsolved Problems in Number Theory, F10.

T. D. Noe, Table of n, a(n) for n=1..10000
Albert Frank, International Contest Of Logical Sequences, 2002 - 2003. Item 4
Albert Frank, Solutions of International Contest Of Logical Sequences, 2002 - 2003.
Peter L. Montgomery, 65-digit solution.

Cf. A036236, A015911, A015921, A001567.

Cf. A000079, A062173, A082495.

import Math.NumberTheory.Moduli (powerMod)
a015910 n = powerMod 2 n n  -- Reinhard Zumkeller, Oct 17 2015

[Modexp(2, n, n): n in [1..100]]; // Vincenzo Librandi, Nov 09 2018

A015910 := n-> modp(2 &^ n,n) ; # Zerinvary Lajos, Feb 15 2008

Mathematica
```
Table[PowerMod[2, n, n], {n, 85}]
```

makelist(power_mod(2, n, n), n, 1, 84); /* Bruno Berselli, May 20 2011 */

a(n)=lift(Mod(2,n)^n) \\ Charles R Greathouse IV, Jul 15 2011

a(2^k) = 0. - Alonso del Arte, Nov 10 2014

a(n) == 2^(n-phi(n)) mod n, where phi(n) = A000010(n). - Thomas Ordowski, Mar 26 2016

A015922 Numbers k such that 2^k == 8 (mod k).

Original entry on oeis.org

1, 2, 3, 4, 8, 9, 15, 21, 33, 39, 51, 57, 63, 69, 87, 93, 111, 123, 129, 141, 159, 177, 183, 195, 201, 213, 219, 237, 248, 249, 267, 291, 303, 309, 315, 321, 327, 339, 381, 393, 399, 411, 417, 447, 453, 471, 489, 501, 519, 537, 543, 573, 579, 591, 597, 633

Offset: 1

Robert G. Wilson v

nonn

For all m, 2^A015921(m) - 1 belongs to this sequence.

Michael De Vlieger, Table of n, a(n) for n = 1..29055 (first 6822 terms from Zak Seidov)
OEIS Wiki, 2^n mod n.

Contains A033553 as a subsequence.
The odd terms form A276967.
Cf. A015921, A130133, A130134.

a015922Q[n_Integer] := If[Mod[2^n, n] == Mod[8, n], True, False];
a015922[n_Integer] :=
Flatten[Position[Thread[a015922Q[Range[n]]], True]];
a015922[1000000] (* Michael De Vlieger, Jul 16 2014 *)
m = 8; Join[Select[Range[m], Divisible[2^# - m, #] &], Select[Range[m + 1, 10^3], PowerMod[2, #, #] == m &]] (* Robert Price, Oct 12 2018 *)
Join[{1,2,3,4,8},Select[Range[650],PowerMod[2,#,#]==8&]] (* Harvey P. Dale, Aug 22 2020 *)

isok(n) = Mod(2, n)^n == Mod(8, n); \\ Michel Marcus, Oct 13 2013, Jul 16 2014

First 5 terms inserted by David W. Wilson

A173572 Odd integers n such that 2^n == 4 (mod n).

Original entry on oeis.org

1, 20737, 93527, 228727, 373457, 540857, 2231327, 11232137, 15088847, 15235703, 24601943, 43092527, 49891487, 66171767, 71429177, 137134727, 207426737, 209402327, 269165561, 302357057, 383696711, 513013327

Offset: 1

Michel Lagneau, Feb 22 2010

nonn

The odd terms of A015921.
Also, nonprime integers n such that 2^(n-2) == 1 (mod n).
For all m, 2^A050259(m)-1 belongs to this sequence.
If n > 1 is a term and p is a primitive prime factor of 2^(n-2)-1, then n*p is also a term. Hence, the sequence is infinite. (Rotkiewicz 1984)

A. E. Bojarincev, Asymptotic expressions for the n-th composite number, Univ. Mat. Zap. 6:21-43 (1967). (in Russian)
R. K. Guy, Unsolved Problems in Number Theory, Springer-Verlag, Third Edition, 2004
G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 3rd ed., Oxford Univ. Press, 1954, p. 2.

Max Alekseyev, Table of n, a(n) for n = 1..722 (all terms below 10^14)
C. K. Caldwell, Composite Numbers
A. Rotkiewicz, On the congruence 2^(n-2) == 1 (mod n). Math. Comp. 43 (1984), 271-272.

Cf. A015921, A050259, A173138, A002808.

with(numtheory): for n from 1 to 100000000 do: a:= 2^(n-2)- 1; b:= a / n; c:= floor(b): if b = c and tau(n) <> 2 then print (n); else fi;od:

m = 4; Join[Select[Range[1, m, 2], Divisible[2^# - m, #] &], Select[Range[m + 1, 10^6, 2], PowerMod[2, #, #] == m &]] (* Robert Price, Oct 12 2018 *)

is(n) = n%2==1 && Mod(2,n)^n==Mod(4,n) \\ Jinyuan Wang, Feb 22 2019

Edited and term 1 prepended by Max Alekseyev, Aug 09 2012

A116630 Positive integers n such that 13^n == 4 (mod n).

Original entry on oeis.org

1, 3, 51, 129, 125869, 158287, 1723647, 1839003, 90808797, 3661886147, 7368982721, 130424652229, 1616928424359, 4003183891851, 66657658685869

Offset: 1

Zak Seidov, Feb 19 2006

No other terms below 10^15. - Max Alekseyev, Nov 26 2017

Some larger terms: 84058689739550643018360088224267, 11083544368708558891212925543084197628431243723. - Max Alekseyev, Jun 26 2011

Solutions to 13^n == k (mod n): A001022 (k=0), A015963(k=-1), A116621 (k=1), A116622 (k=2), A116629(k=3), this sequence (k=4), A116611 (k=5), A116631 (k=6), A116632 (k=7), A295532 (k=8), A116636 (k=9), A116620(k=10), A116638 (k=11), A116639 (k=15)

Cf. A116609, A015921, A125949.

Join[{1, 3}, Select[Range[1, 5000], Mod[13^#, #] == 4 &]] (* G. C. Greubel, Nov 19 2017 *)
Join[{1, 3}, Select[Range[2000000], PowerMod[13, #, #] == 4 &]] (* Robert Price, Apr 10 2020 *)

isok(n) = Mod(13, n)^n == 4; \\ Michel Marcus, Nov 19 2017

More terms from Ryan Propper, Jan 09 2008

Terms 1,3 prepended and a(12)-a(15) added by Max Alekseyev, Jun 26 2011, Nov 26 2017

A128122 Numbers m such that 2^m == 6 (mod m).

Original entry on oeis.org

1, 2, 10669, 6611474, 43070220513807782

Offset: 1

Alexander Adamchuk, Feb 15 2007

No other terms below 10^17. - Max Alekseyev, Nov 18 2022

A large term: 862*(2^861-3)/281437921287063162726198552345362315020202285185118249390789 (203 digits). - Max Alekseyev, Sep 24 2016

			2 == 6 (mod 1), so 1 is a term;
4 == 6 (mod 2), so 2 is a term.

Joe K. Crump, 2^n mod n
OEIS Wiki, 2^n mod n

Solutions to 2^m == k (mod m): A000079 (k=0),A187787 (k=1/2), A296369 (k=-1/2), A006521 (k=-1), A296370 (k=3/2), A015919 (k=2), A006517 (k=-2), A050259 (k=3), A015940 (k=-3), A015921 (k=4), A244673 (k=-4), A128121 (k=5), A245318 (k=-5), this sequence (k=6), A245728 (k=-6), A033981 (k=7), A240941 (k=-7), A015922 (k=8), A245319 (k=-8), A051447 (k=9), A240942 (k=-9), A128123 (k=10), A245594 (k=-10), A033982 (k=11), A128124 (k=12), A051446 (k=13), A128125 (k=14), A033983 (k=15), A015924 (k=16), A124974 (k=17), A128126 (k=18), A125000 (k=19), A015925 (k=2^5), A015926 (k=2^6), A015927 (k=2^7), A015929 (k=2^8), A015931 (k=2^9), A015932 (k=2^10), A015935 (k=2^11), A015937 (k=2^12)

Cf. A015910, A036236.

m = 6; Join[Select[Range[m], Divisible[2^# - m, #] &],
Select[Range[m + 1, 10^6], PowerMod[2, #, #] == m &]] (* Robert Price, Oct 08 2018 *)

1 and 2 added by N. J. A. Sloane, Apr 23 2007

a(5) from Max Alekseyev, Nov 18 2022

A296369 Numbers m such that 2^m == -1/2 (mod m).

Original entry on oeis.org

1, 5, 65, 377, 1189, 1469, 25805, 58589, 134945, 137345, 170585, 272609, 285389, 420209, 538733, 592409, 618449, 680705, 778805, 1163065, 1520441, 1700945, 2099201, 2831009, 4020029, 4174169, 4516109, 5059889, 5215769

Offset: 1

Max Alekseyev, Dec 10 2017

nonn

Equivalently, 2^(m+1) == -1 (mod m), or m divides 2^(m+1) + 1.

The sequence is infinite, see A055685.

Chai Wah Wu, Table of n, a(n) for n = 1..1192
OEIS Wiki, 2^n mod n

Solutions to 2^m == k (mod m): A296370 (k=3/2), A187787 (k=1/2), this sequence (k=-1/2), A000079 (k=0), A006521 (k=-1), A015919 (k=2), A006517 (k=-2), A050259 (k=3), A015940 (k=-3), A015921 (k=4), A244673 (k=-4), A128121 (k=5), A245318 (k=-5), A128122 (k=6), A245728 (k=-6), A033981 (k=7), A240941 (k=-7), A015922 (k=8), A245319 (k=-8), A051447 (k=9), A240942 (k=-9), A128123 (k=10), A245594 (k=-10), A033982 (k=11), A128124 (k=12), A051446 (k=13), A128125 (k=14), A033983 (k=15), A015924 (k=16), A124974 (k=17), A128126 (k=18), A125000 (k=19), A015925 (k=2^5), A015926 (k=2^6), A015927 (k=2^7), A015929 (k=2^8), A015931 (k=2^9), A015932 (k=2^10), A015935 (k=2^11), A015937 (k=2^12)

Select[Range[10^5], Divisible[2^(# + 1) + 1, #] &] (* Robert Price, Oct 11 2018 *)

A296369_list = [n for n in range(1,10**6) if pow(2,n+1,n) == n-1] # Chai Wah Wu, Nov 04 2019

a(n) = A055685(n) - 1.

Incorrect term 4285389 removed by Chai Wah Wu, Nov 04 2019

A015926 Positive integers n such that 2^n == 2^6 (mod n).

Original entry on oeis.org

1, 2, 4, 6, 8, 10, 12, 16, 18, 24, 30, 31, 32, 36, 42, 48, 64, 66, 72, 78, 84, 90, 96, 102, 114, 126, 138, 144, 168, 174, 176, 186, 192, 210, 222, 234, 246, 252, 258, 282, 288, 318, 336, 354, 366, 390, 396, 402, 426, 438, 456, 474, 496, 498, 504, 510, 534, 546

Offset: 1

Robert G. Wilson v

nonn

The odd terms are given by A215610.

For all m, 2^A033981(m)-1 belongs to this sequence.

T. D. Noe, Table of n, a(n) for n = 1..1000
OEIS Wiki, 2^n mod n

Contains A208154 as a subsequence.

Cf. A015919, A015921, A015922, A015924, A015925, A015927, A015929, A015931, A015932, A015935, A015937.

Select[Range[1000], Mod[2^# - 2^6, #] == 0 &] (* T. D. Noe, Aug 17 2012 *)

Edited by Max Alekseyev, Jul 30 2011

A015929 Positive integers n such that 2^n == 2^8 (mod n).

Original entry on oeis.org

1, 2, 4, 6, 8, 12, 14, 16, 20, 24, 32, 40, 48, 56, 60, 64, 80, 88, 96, 104, 120, 127, 128, 136, 140, 152, 160, 184, 192, 224, 232, 240, 248, 256, 260, 272, 296, 308, 320, 328, 344, 376, 384, 408, 416, 424, 472, 480

Offset: 1

Robert G. Wilson v

nonn

The odd terms are given by A215611.

For all m, 2^A051447(m)-1 belongs to this sequence.

T. D. Noe, Table of n, a(n) for n = 1..1000
OEIS Wiki, 2^n mod n

Contains A208156 as a subsequence.

Cf. A015919, A015921, A015922, A015924, A015925, A015926, A015927, A015931, A015932, A015935, A015937.

Select[Range[1000], Mod[2^# - 2^8, #] == 0 &] (* T. D. Noe, Aug 17 2012 *)

Edited by Max Alekseyev, Jul 30 2011

A015932 Positive integers n such that 2^n == 2^10 (mod n).

Original entry on oeis.org

1, 2, 4, 6, 7, 8, 10, 12, 16, 24, 28, 30, 32, 34, 48, 50, 64, 70, 73, 96, 110, 112, 128, 130, 150, 170, 190, 192, 230, 256, 290, 310, 330, 370, 384, 410, 430, 442, 448, 470, 512, 530, 532, 550, 590, 610, 670, 710

Offset: 1

Robert G. Wilson v

nonn

The odd terms are given by A215612.

For all m, 2^A033982(m)-1 belongs to this sequence.

T. D. Noe, Table of n, a(n) for n = 1..1000
OEIS Wiki, 2^n mod n

Contains A208158 as a subsequence.

Cf. A015919, A015921, A015922, A015924, A015925, A015926, A015927, A015929, A015931, A015935, A015937.

Select[Range[1000], Mod[2^# - 2^10, #] == 0 &] (* T. D. Noe, Aug 17 2012 *)

Edited by Max Alekseyev, Jul 30 2011

A015937 Positive integers n such that 2^n == 2^12 (mod n).

Original entry on oeis.org

1, 2, 4, 6, 8, 12, 16, 18, 20, 22, 23, 24, 32, 36, 40, 42, 48, 60, 62, 64, 68, 72, 80, 84, 89, 96, 120, 126, 128, 132, 144, 156, 160, 168, 180, 192, 204, 228, 240, 252, 256, 276, 288, 312, 320, 336, 340, 348, 352, 360, 372, 384, 420, 444, 462

Offset: 1

Robert G. Wilson v

nonn

The odd terms are given by A215613.

For all m, 2^A051446(m)-1 belongs to this sequence.

T. D. Noe, Table of n, a(n) for n = 1..1000
OEIS Wiki, 2^n mod n

Cf. A015919, A015921, A015922, A015924, A015925, A015926, A015927, A015929, A015931, A015932, A015935.

With[{c=2^12},Select[Range[1,6000],Divisible[2^#-c,#]&]] (* Harvey P. Dale, Mar 20 2011 *)

Edited by Max Alekseyev, Jul 31 2011

cp's OEIS Frontend

A015910 a(n) = 2^n mod n.

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A015922 Numbers k such that 2^k == 8 (mod k).

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A173572 Odd integers n such that 2^n == 4 (mod n).

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A116630 Positive integers n such that 13^n == 4 (mod n).

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A128122 Numbers m such that 2^m == 6 (mod m).

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A296369 Numbers m such that 2^m == -1/2 (mod m).

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A015926 Positive integers n such that 2^n == 2^6 (mod n).

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