Return True if x is a fractional ideal of a relative number field.
EXAMPLES:
sage: is_NumberFieldFractionalIdeal_rel(2/3)
False
sage: is_NumberFieldFractionalIdeal_rel(ideal(5))
False
sage: k.<a> = NumberField(x^2 + 2)
sage: I = k.ideal([a + 1]); I
Fractional ideal (a + 1)
sage: is_NumberFieldFractionalIdeal_rel(I)
False
sage: R.<x> = QQ[]
sage: K.<a> = NumberField(x^2+6)
sage: L.<b> = K.extension(K['x'].gen()^4 + a)
sage: I = L.ideal(b); I
Fractional ideal (b)
sage: is_NumberFieldFractionalIdeal_rel(I)
True
sage: N = I.norm(); N
Fractional ideal (-a)
sage: is_NumberFieldFractionalIdeal_rel(N)
False
sage: is_NumberFieldFractionalIdeal(N)
True
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