Strobogrammatic number Strobogrammatic number A strobogrammatic number is a number whose numeral is rotationally symmetric, so that it appears the same when rotated 180 degrees. In other words, the numeral looks the same right-side up and upside down (e.g., 69, 96, 1001). A strobogrammatic prime is a strobogrammatic number that is also a prime number, i.e., a number that is only divisible by one and itself (e.g., 11). It is a type of ambigram, words and numbers that retain their meaning when viewed from a different perspective, such as palindromes. When written using standard characters (ASCII), the numbers, 0, 1, 8 are
Plane at infinity does make it look "special" in comparison to the other planes of the space. If the affine 3-space is real, formula_2, then the addition of a real projective plane formula_3 at infinity produces the real projective 3-space formula_4. Since any two projective planes in a projective 3-space are equivalent, we can choose a homogeneous coordinate system so that any point on the plane at infinity is represented as ("X":"Y":"Z":0). Any point in the affine 3-space will then be represented as ("X":"Y":"Z":1). The points on the plane at infinity seem to have three degrees of freedom, but homogeneous coordinates are equivalent