Q. Curve \(y^2 = x^6 + 2x^3 + 4x^2 + 4x + 1\).

Answer

To find integer solutions for the curve \(y^2 = x^6 + 2x^3 + 4x^2 + 4x + 1\), compare the polynomial to the perfect square \((x^3 + 1)^2 = x^6 + 2x^3 + 1\).

The equation can be rewritten as:
\[
y^2 = (x^3 + 1)^2 + 4x(x + 1)
\]

For the expression to be a perfect square \(y^2\), the term \(4x(x + 1)\) must satisfy specific conditions.

1. If \(x = 0\), then \(y^2 = (0+1)^2 + 0 = 1\), giving solutions \((0,1)\) and \((0,-1)\).
2. If \(x = -1\), then \(y^2 = (-1+1)^2 + 0 = 0\), giving solution \((-1,0)\).

For other integer values of \(x\), the term \(4x(x + 1)\) is strictly positive, placing \(x^6 + 2x^3 + 4x^2 + 4x + 1\) between consecutive perfect squares for large \(|x|\), which limits further integer solutions.

Final integer solutions \((x,y)\): \((0,1),\ (0,-1),\ (-1,0)\).

Detailed Explanation

Find all integer solutions for the curve \(y^2 = x^6 + 2x^3 + 4x^2 + 4x + 1\)

  1. Step 1 – Compare the expression to a known perfect square:

    Notice that the first two terms \(x^6 + 2x^3\) are part of the expansion of \((x^3 + 1)^2\). We can expand this square to compare:
    \[
    (x^3 + 1)^2 = x^6 + 2x^3 + 1.
    \]
    The difference between the given curve and this square is:
    \[
    (x^6 + 2x^3 + 4x^2 + 4x + 1) – (x^6 + 2x^3 + 1) = 4x^2 + 4x = 4x(x + 1).
    \]

  2. Step 2 – Identify intervals where the expression lies between squares:

    If \(x>0\) or \(x<-1\), the product \(4x(x + 1)\) is strictly positive. This means for these values: \[ y^2 > (x^3 + 1)^2.
    \]
    Now compare the expression to the next consecutive square, \((x^3 + 2)^2\):
    \[
    (x^3 + 2)^2 = x^6 + 4x^3 + 4.
    \]
    Subtracting our expression from this gives:
    \[
    (x^6 + 4x^3 + 4) – (x^6 + 2x^3 + 4x^2 + 4x + 1) = 2x^3 – 4x^2 – 4x + 3.
    \]
    For \(x\ge 3\), this difference is positive, meaning
    \[
    y^2 < (x^3 + 2)^2. \] When an integer \(y^2\) is strictly between two consecutive squares, it cannot be a perfect square itself.

  3. Step 3 – Test the remaining integer candidates:

    Because the bounding logic excludes \(x\ge 3\) and \(x\le -2\), we only need to manually check the integers \(x = -1, 0, 1, 2\).

  4. Step 4 – Evaluate \(x = -1\):

    \(y^2 = (-1)^6 + 2(-1)^3 + 4(-1)^2 + 4(-1) + 1\)

    \(y^2 = 1 – 2 + 4 – 4 + 1 = 0\).

    Thus \(y = 0\). This gives the solution \((-1, 0)\).

  5. Step 5 – Evaluate \(x = 0\):

    \(y^2 = 0^6 + 2\cdot0^3 + 4\cdot0^2 + 4\cdot0 + 1 = 1\).

    Thus \(y = \pm 1\). This gives the solutions \((0, 1)\) and \((0, -1)\).

  6. Step 6 – Evaluate \(x = 1\) and \(x = 2\):

    For \(x = 1\): \(y^2 = 1 + 2 + 4 + 4 + 1 = 12\) (not a square).

    For \(x = 2\): \(y^2 = 64 + 16 + 16 + 8 + 1 = 105\) (not a square).

  7. Final Answer:

    The integer solutions are \((0, 1)\), \((0, -1)\), and \((-1, 0)\).

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FAQs

Can the polynomial in \(y^2 = x^6 + 2x^3 + 4x^2 + 4x + 1\) be factored over \(\mathbb{Q}\) or is it a perfect square?

Check for rational roots and use polynomial-factor tests or CAS. It is not a perfect square (no cubic squared yields those coefficients). computer algebra system is recommended to decide irreducibility over \(\mathbb{Q}\); heuristically it is likely irreducible over \(\mathbb{Q}\).

Is this a hyperelliptic curve and what is its genus?

Yes: \(y^2 = f(x)\) with \(\deg f = 6\) gives a hyperelliptic curve of genus \(g = (6-2)/2 = 2\) (equivalently \(g = 2\) for degree \(2g+2\)), provided \(f(x)\) has distinct roots (i.e., the curve is smooth).

Are there singular points on the curve?

Test singularities by computing \(\gcd(f,f')\) where \(f(x)=x^6+2x^3+4x^2+4x+1\) and \(f'(x)=6x^5+6x^2+8x+4\). If the gcd is 1, the curve is smooth. Practically compute with CAS; generically this polynomial is separable so the curve is nonsingular.

Does the curve have obvious rational or integer points?

Yes: \(x=0\) gives \(y^2=1\), so \((0,\pm1)\) are integer points. Also \(x=-1\) gives \(y=0\), so \((-1,0)\) lies on the curve. For more rational points use a bounded search and genus-2 methods (Chabauty, descent) -- there are finitely many by Faltings.

Can this genus-2 curve be parametrized or birationally transformed to an elliptic curve?

No rational parametrization exists for a genus-2 curve. It cannot be birational to an elliptic curve because genus is a birational invariant; genus-2 cannot be transformed to genus-1.

How many real components does the real locus have and what is the behavior as \(x\to\pm\infty\)?

How many real components does the real locus have and what is the behavior as \(x\to\pm\infty\)?

What methods can I use to find all rational points on this curve?

Use a combination: search small integers, compute the Jacobian and its rank, apply Chabauty-Coleman if rank < genus, or 2-descent and Mordell-Weil sieves. Implementations in Sage, Magma, or PARI/GP are standard tools.

How should I visualize or plot the curve \(y^2=f(x)\)?

Plot the two real branches \(y=\pm\sqrt{f(x)}\) where \(f(x)\ge0\). Use numeric root-finding to locate zeros of \(f\) and sample intervals. Software: Sage, Mathematica, Python (matplotlib + numpy), or Desmos for quick visuals.
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