# Schizophrenic number

A schizophrenic number or mock rational number is an irrational number which displays certain characteristics of rational numbers. It is one of the numerous mathematical curiosities.

## Definition

The Universal Book of Mathematics defines "schizophrenic number" as:
   An informal name for an irrational number that displays such persistent patterns in its decimal expansion, that it has the appearance of a rational number. A schizophrenic number can be obtained as follows. For any positive integer n, let f (n) denote the integer given by the recurrence f (n) = 10 f (n − 1) + n with the initial value f(0) = 0. Thus, f (1) = 1, f (2) = 12, f (3) = 123, and so on. The square roots of f (n) for odd integers n give rise to a curious mixture appearing to be rational for periods, and then disintegrating into irrationality. This is illustrated by the first 500 digits of √f (49):
   The repeating strings become progressively shorter and the scrambled strings become larger until eventually the repeating strings disappear. However, by increasing n we can forestall the disappearance of the repeating strings as long as we like. The repeating digits are always 1, 5, 6, 2, 4, 9, 6, 3, 9, 2, ... .
The sequence of numbers generated by the recurrence relation f (n) = 10 f (n − 1) + n described above is:
    0, 1, 12, 123, 1234, 12345, 123456, 1234567, 12345678, 123456789, 1234567900, ... (sequence A014824 in the OEIS).
    f(49) = 1234567901234567901234567901234567901234567901229
The integer parts of their square roots,
    1, 3, 11, 35, 111, 351, 1111, 3513, 11111, 35136, 111111, 351364, 1111111, ... (sequence A068995 in the OEIS),
alternate between numbers with irregular digits and numbers with repeating digits, in a similar way to the alternations appearing within the decimal part of each square root.

## Characteristics

The schizophrenic number shown above is the special case of a more general phenomenon that appears in the $b$-ary expansions of square roots of the solutions of the recurrence $f_{b}(n)=bf_{b}(n-1)+n$, for all $b\geq 2$, with initial value $f(0)=0$ taken at odd positive integers $n$. The case $b=10$ and $n=49$ corresponds to the example above.
Indeed, Tóth showed that these irrational numbers present schizophrenic patterns within their $b$-ary expansion, composed of blocks that begin with a non-repeating digit block followed by a repeating digit block. When put together in base $b$, these blocks form the schizophrenic pattern. For instance, in base 8, the number √(f$_8$(49)){\textstyle {\sqrt {f_{8}(49)}}} begins:
The pattern is due to the Taylor expansion of the square root of the recurrence's solution taken at odd positive integers. The various digit contributions of the Taylor expansion yield the non-repeating and repeating digit blocks that form the schizophrenic pattern.

## Other properties

In some cases, instead of repeating digit sequences, we find repeating digit patterns. For instance, the number √(f$_3$(49)){\textstyle {\sqrt {f_{3}(49)}}}:
shows repeating digit patterns in base $3$.
Numbers that are schizophrenic in base $b$ are also schizophrenic in base $b^{m}$, up to a certain limit (see Tóth). An example is √(f$_3$(49)){\textstyle {\sqrt {f_{3}(49)}}} above, which is still schizophrenic in base $9$:

## History

Clifford A. Pickover has said that the schizophrenic numbers were discovered by Kevin Brown. In Wonders of Numbers Pickover described the history of schizophrenic numbers thus:
   The construction and discovery of schizophrenic numbers was prompted by a claim (posted in the Usenet newsgroup sci.math) that the digits of an irrational number chosen at random would not be expected to display obvious patterns in the first 100 digits. It was said that if such a pattern were found, it would be irrefutable proof of the existence of either God or extraterrestrial intelligence. (An irrational number is any number that cannot be expressed as a ratio of two integers. Transcendental numbers like e and π, and noninteger surds such as square root of 2 are irrational.)