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## 2.9 Referencing results and special numbers

An `@` used for an escape character; rules currently in force are as follows.

`@n`

The evaluated result of n-th input command

`@@`

The evaluated result of the last command

`@i`

The unit of imaginary number, square root of -1.

`@pi`

The number pi, the ratio of a circumference of the circle and its diameter.

`@e`

Napier’s number, the base of natural logarithm.

`@`

A generator of GF(2^m), a finite field of characteristic 2, over GF(2). It is a root of an irreducible univariate polynomial over GF(2) which is set as the defining polynomial of GF(2^m).

`@>, @<, @>=, @<=, @==, @&&, @||`

Fist order logical operators. They are used in quantifier elimination.

``` fctr(x^10-1);
[[1,1],[x-1,1],[x+1,1],[x^4+x^3+x^2+x+1,1],[x^4-x^3+x^2-x+1,1]]
 @@;
[x^4+x^3+x^2+x+1,1]
 eval(sin(@pi/2));
1.000000000000000000000000000000000000000000000000000000000
 eval(log(@e),20);
0.99999999999999999999999999998
 @0;
x^4-x^3+x^2-x+1
 (1+@i)^5;
(-4-4*@i)
 eval(exp(@pi*@i));
-1.0000000000000000000000000000
 (@+1)^9;
(@^9+@^8+@+1)
```

As you can see in the above example, results of toplevel computation can be referred to by `@` convention. This is convenient for users, while it sometimes imposes a heavy burden to the garbage collector. It may happen that GC time will rapidly increase after computing a very large expression at the toplevel. In such cases `delete_history()` (see section `delete_history`) takes effect.

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