Quadratic Residues and the Law of Reciprocity · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Which remainders are squares

Mathematics · Number Theory · ages 21-22
Name ______________________   Date ____________
  1. Let g be primitive modulo 11. How many distinct nonzero remainders occur among g^0, g^2, g^4, g^6 and g^8?

    Answer: ______________

  2. Which set contains all remainders x with x^2 congruent to 4 modulo 7?

    • {2, 4}
    • {3, 4}
    • {2, 5}
    • {1, 6}
  3. Modulo 5, which pair lists all nonzero squares?

    • {1, 2}
    • {1, 4}
    • {2, 3}
  4. Modulo 7, the number 2 is a square because 3 squared leaves remainder 2. True or false?

    Circle one:   True   False

  5. Use the residue-count formula to find the number of nonzero squares modulo 19.

    Answer: ______________

  6. For p = 2, the exponent (p - 1)/2 is an integer, so the stated odd-prime form of Euler's criterion applies. Is this true?

    Circle one:   True   False

  7. Use Euler's criterion to decide whether 7 is a square modulo 11.

    • 7 is a nonsquare modulo 11
    • 7 is a nonzero square modulo 11
    • 7 is divisible by the prime 11
    • 7 has no remainder modulo 11
  8. Modulo 7, 2 raised to the power 3 leaves remainder 1. What does Euler's criterion say about 2?

    • 2 is a quadratic nonresidue
    • 2 is a quadratic residue
    • The criterion gives no answer here
  9. Without computing powers, why is 2 a square modulo 23?

    • 23 leaves 7 modulo 8, so (2/23) = 1
    • 23 leaves 3 modulo 4, so (2/23) = -1
    • 23 is odd and prime, so (2/23) = 0
  10. Use quadratic reciprocity to evaluate (3/29). Enter the Legendre symbol's value.

    Answer: ______________

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Answer key

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Which remainders are squares W1-mt_0KS9eTCY4p-s1

  1. 5 · The generator has order 10. The five displayed exponent classes are distinct modulo 10, so these powers give five distinct squares.
  2. {2, 5} · 2^2 gives 4 modulo 7. The paired root is 7 - 2 = 5. A nonzero square has exactly these two roots.
  3. {1, 4} · Squaring 1, 2, 3, 4 leaves 1, 4, 4, 1, so only 1 and 4 appear.
  4. True · Three squared is 9, and 9 minus 7 leaves 2.
  5. 9 · 19 is an odd prime, so the number is (19 - 1)/2 = 9.
  6. False · For p = 2, the exponent is 1/2, not an integer. The stated criterion requires an odd prime.
  7. 7 is a nonsquare modulo 11 · Euler's exponent is 5. The remainder of 7^5 is 10, representing -1 modulo 11, so 7 is a nonsquare.
  8. 2 is a quadratic residue · Remainder 1 from that power signals a square.
  9. 23 leaves 7 modulo 8, so (2/23) = 1 · 23 = 2 times 8 + 7. The supplementary law for 2 gives symbol value 1 when the prime is 7 modulo 8.
  10. -1 · 29 is 1 modulo 4, so (3/29) = (29/3) = (2/3) = -1.
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