**YY sequence** or **Yabai Y sequence** is a difference sequence system introduced by a Japanese googologist Yukito on 17/07/2020.^{[1]}^{[2]} It is intended to be a notation stronger than Y sequence. Although it has not been formalised yet, the expression \((1,2,5)\) in YY sequence is expected to correspond to the limit of Bashicu matrix system version 2.3 and the expression \((1,3)\) in Y sequence, and the expression \((1,3)\) in YY sequence corresponds to the limit of Y sequence.

## Convention

In order to distinguish a sequence in YY sequence and other sequence notations, people frequently put a shorthand of the notation in front of the expression. For example, (1,3) in Y sequence is denoted by Y(1,3), (1,3) in YY sequence is denoted by YY(1,3), and so on.

As Y sequence allows ω as an entry only when we consider the limit expression Y(1,ω), the limit expression of YY sequence is denoted by YY(1,ω). It does not mean that Y sequence and YY sequence are extended to be systems of sequences of transfinite entries.

## Extensions

Yukito also indicated the existence of higher extensions on 19/07/2020: YYY sequence, YYYY sequence, and Y^{ω} sequence.^{[2]} According to Yukito, there is a sequence "Y^{1} sequence, Y^{2} sequence, Y^{3} sequence, …" such that Y^{1} sequence is Y sequence, Y^{2} sequence is YY sequence, Y^{3} sequence is YYY sequence, Y^{4} sequence is YYYY sequence, and the limit of Y^{n}(1,ω) with n<ω "coincides" with Y^{ω}(1,3).

The last equality probably means that there exists a correspondence from standard expressions in Y^{n} sequence to ordinals for any ordinals n≦ω such for which expansions give fundamental sequences and the limit of ordinals corresponding to Y^{n}(1,ω) coincides with the ordinal corresponding to Y^{ω}(1,3). Although none of the systems has been formalised, it means that the whose system is expected to be much stronger than Y sequence, which is expected to be much stronger than BMS version 2.3.

Although there was a table of analysis among BMS, Y sequence, YY sequence, YYY sequence, and YYYY sequence which tells us how higher extension outgrows YY sequence, it has been deleted since 19/07/2020. According to Yukito, he found a bug in Y^{n} sequence for all n≧3.

Nayuta mentioned that Y^{n} sequence employs a similar idea to an unborn version of N primitive called NΩ.0, and some weird property of YYY(1,3,3) implies that NΩ.0 might not work as intended. Therefore NΩ.0 is canceled.

## Relation to Bell numbers

A Japanese googologist koteitan^{[3]} discovered that the sequence YY(1,2,5,15,52,203) appearing in the expansion of YY(1,3) is an initial segment of the sequence of Bell numbers.^{[4]}^{[5]}^{[6]} Yukito agreed that the expansion of YY(1,3) coincides with the sequence of Bell numbers. According to Yukito, he actually knew Bell numbers but the equality is a coincidence.

## Sources

- ↑ The user page of Yukito in the Japanese Googology Wiki.
- ↑
^{2.0}^{2.1}Yukito,*YYvsY*, Google Spreadsheet. - ↑ The user page of koteitan in Googology Wiki.
- ↑ koteitan, Relation between YY sequence and Bell numbers.
- ↑ Bell numbers, A0000110 - OEIS.
- ↑ E. T. Bell,
*Exponential polynomials*], Annals of Mathematics, Second Series, Vol. 35, No. 2 (1934), pp. 258--277.

## See also

**Fish numbers:** Fish number 1 · Fish number 2 · Fish number 3 · Fish number 4 · Fish number 5 · Fish number 6 · Fish number 7**Mapping functions:** S map · SS map · S(n) map · M(n) map · M(m,n) map**By Aeton:** Okojo numbers · N-growing hierarchy**By BashicuHyudora:** Primitive sequence number · Pair sequence number · Bashicu matrix system**By Kanrokoti:** KumaKuma ψ function**By 巨大数大好きbot:** Flan numbers**By Jason:** Irrational arrow notation · δOCF · δφ · ε function**By mrna:** 段階配列表記 · 降下段階配列表記 · 多変数段階配列表記 · SSAN · S-σ**By Nayuta Ito:** N primitive**By p進大好きbot:** Large Number Garden Number**By Yukito:** Hyper primitive sequence system · Y sequence · **YY sequence** · Y function**Indian counting system:** Lakh · Crore · Tallakshana · Uppala · Dvajagravati · Paduma · Mahakathana · Asankhyeya · Dvajagranisamani · Vahanaprajnapti · Inga · Kuruta · Sarvanikshepa · Agrasara · Uttaraparamanurajahpravesa · Avatamsaka Sutra · Nirabhilapya nirabhilapya parivarta**Chinese, Japanese and Korean counting system:** Wan · Yi · Zhao · Jing · Gai · Zi · Rang · Gou · Jian · Zheng · Zai · Ji · Gougasha · Asougi · Nayuta · Fukashigi · Muryoutaisuu**Other:** Taro's multivariable Ackermann function · TR function · Arai's \(\psi\) · *Sushi Kokuu Hen*