how is this hash probing method quadratic?

I'm having a problem distinguishing between quadratic and linear probing algorithms. When I'm reading conceptual explanations, I see I^2 being repeatedly added to the last index tried. How is that the case here? What would linear probing change this into? From what I'm reading, the method below implements quadratic probing.

private int findPosQuadratic( AnyType x )
{
    int offset = 1;
    int currentPos = myhash( x );

    while( array[ currentPos ] != null &&
            !array[ currentPos ].element.equals( x ) )
    {
        currentPos += offset;  // Compute ith probe
        offset += 2;
        if( currentPos >= array.length )
            currentPos -= array.length;
    }

    return currentPos;
}

The indices generated here are:

H // offset : 1

H + 1 // offset : 3

H + 4 // offset : 5

H + 9 // offset : 7

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Since n ^ 2 = 1 + 3 + 5 + ... + (n * 2 - 1), this is actually the function H(n) = H + n ^ 2, so it is quadratic probing.