2^(10^10) mod 101 is?

2 1 0 10 m o d 101 2^{10^{10}} \bmod 101


The answer is 1.

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4 solutions

Otto Bretscher
Dec 13, 2018

2 1 0 10 = ( 2 100 ) 1 0 8 1 1 0 8 = 1 ( m o d 101 ) 2^{10^{10}}=(2^{100})^{10^8}\equiv 1^{10^8}=1 \pmod{101} by Fermat's little theorem.

Jordan Cahn
Dec 13, 2018

Let 101 \equiv_{101} denote congruence m o d 101 \bmod 101 . Then, by Fermat's Little Theorem ,

2 1 0 10 101 2 ( 1 0 10 m o d 100 ) 101 2 ( 10 0 5 m o d 100 ) 101 2 0 101 1 \begin{aligned} 2^{10^{10}} &\equiv_{101} 2^{\left(10^{10}\bmod 100\right)} \\ &\equiv_{101} 2^{\left(100^5 \bmod 100\right)} \\ &\equiv_{101} 2^0 \\ &\equiv_{101} \boxed{1} \end{aligned}

Aaghaz Mahajan
Dec 13, 2018

Ummm............a direct application of Fermat's Little Theorem would suffice.........

Compute consecutive powers of 2 2 until its modulus 101 101 is 1 or the problem is solved by brute force.

N.B., when the power of 2 2 becomes larger than 101 101 only the modulus needs to be retained.

( 1 2 2 4 3 8 4 16 5 32 6 64 7 27 8 54 9 7 10 14 11 28 12 56 13 11 14 22 15 44 16 88 17 75 18 49 19 98 20 95 21 89 22 77 23 53 24 5 25 10 26 20 27 40 28 80 29 59 30 17 31 34 32 68 33 35 34 70 35 39 36 78 37 55 38 9 39 18 40 36 41 72 42 43 43 86 44 71 45 41 46 82 47 63 48 25 49 50 50 100 51 99 52 97 53 93 54 85 55 69 56 37 57 74 58 47 59 94 60 87 61 73 62 45 63 90 64 79 65 57 66 13 67 26 68 52 69 3 70 6 71 12 72 24 73 48 74 96 75 91 76 81 77 61 78 21 79 42 80 84 81 67 82 33 83 66 84 31 85 62 86 23 87 46 88 92 89 83 90 65 91 29 92 58 93 15 94 30 95 60 96 19 97 38 98 76 99 51 100 1 ) \left( \begin{array}{cc} 1 & 2 \\ 2 & 4 \\ 3 & 8 \\ 4 & 16 \\ 5 & 32 \\ 6 & 64 \\ 7 & 27 \\ 8 & 54 \\ 9 & 7 \\ 10 & 14 \\ 11 & 28 \\ 12 & 56 \\ 13 & 11 \\ 14 & 22 \\ 15 & 44 \\ 16 & 88 \\ 17 & 75 \\ 18 & 49 \\ 19 & 98 \\ 20 & 95 \\ 21 & 89 \\ 22 & 77 \\ 23 & 53 \\ 24 & 5 \\ 25 & 10 \\ 26 & 20 \\ 27 & 40 \\ 28 & 80 \\ 29 & 59 \\ 30 & 17 \\ 31 & 34 \\ 32 & 68 \\ 33 & 35 \\ 34 & 70 \\ 35 & 39 \\ 36 & 78 \\ 37 & 55 \\ 38 & 9 \\ 39 & 18 \\ 40 & 36 \\ 41 & 72 \\ 42 & 43 \\ 43 & 86 \\ 44 & 71 \\ 45 & 41 \\ 46 & 82 \\ 47 & 63 \\ 48 & 25 \\ 49 & 50 \\ 50 & 100 \\ 51 & 99 \\ 52 & 97 \\ 53 & 93 \\ 54 & 85 \\ 55 & 69 \\ 56 & 37 \\ 57 & 74 \\ 58 & 47 \\ 59 & 94 \\ 60 & 87 \\ 61 & 73 \\ 62 & 45 \\ 63 & 90 \\ 64 & 79 \\ 65 & 57 \\ 66 & 13 \\ 67 & 26 \\ 68 & 52 \\ 69 & 3 \\ 70 & 6 \\ 71 & 12 \\ 72 & 24 \\ 73 & 48 \\ 74 & 96 \\ 75 & 91 \\ 76 & 81 \\ 77 & 61 \\ 78 & 21 \\ 79 & 42 \\ 80 & 84 \\ 81 & 67 \\ 82 & 33 \\ 83 & 66 \\ 84 & 31 \\ 85 & 62 \\ 86 & 23 \\ 87 & 46 \\ 88 & 92 \\ 89 & 83 \\ 90 & 65 \\ 91 & 29 \\ 92 & 58 \\ 93 & 15 \\ 94 & 30 \\ 95 & 60 \\ 96 & 19 \\ 97 & 38 \\ 98 & 76 \\ 99 & 51 \\ 100 & 1 \\ \end{array} \right)

Rewrite 2 1 0 10 2^{10^{10}} as 2 10 0 5 2^{100^5} .

The problem can now be rewritten as ( 1 5 m o d 101 ) (1^5 \bmod 101) .

The answer is 1

Touché! I learned something, thanks to both of you.

A Former Brilliant Member - 2 years, 6 months ago

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