One OEIS after another

Question

As of 13/03/2018 16:45 UTC, the winner is answer #345, by Khuldraeseth na'Barya. This means the contest is officially over, but feel free to continue posting answers, just so long as they follow the rules.

As well, just a quick shout out to the top three answerers in terms of numbers of answers:

1. NieDzejkob - 41 answers

2. KSmarts - 30 answers

3. Hyper Neutrino - 26 answers

This is an answer chaining question that uses sequences from OEIS, and the length of the previous submission.

This answer chaining question will work in the following way:

I will post the first answer. All other solutions must stem from that.
The next user (let's call them userA) will find the OEIS sequence in which its index number (see below) is the same as the length of my code.
Using the sequence, they must then code, in an unused language, a program that takes an integer as input, n, and outputs the nth number in that sequence.
Next, they post their solution after mine, and a new user (userB) must repeat the same thing.

The nth term of a sequence is the term n times after the first, working with the first value being the first value given on its OEIS page. In this question, we will use 0-indexing for these sequences. For example, with A000242 and n = 3, the correct result would be 25.

However!

This is not a code-golf, so shortest code doesn't matter. But the length of your code does still have an impact. To prevent the duplication of sequences, your bytecount must be unique. This means that no other program submitted here can be the same length in bytes as yours.

If there isn't a sequence for then length of the last post, then the sequence for your post is the lowest unused sequence. This means that the sequences used also have to be unique, and that the sequence cannot be the same as your bytecount.

After an answer has been posted and no new answers have been posted for more than a week, the answer before the last posted (the one who didn't break the chain) will win.

Input and Output

Generic input and output rules apply. Input must be an integer or a string representation of an integer and output must be the correct value in the sequence.

Formatting

As with most answer-chaining questions, please format your answer like this

# N. language, length, [sequence](link)

`code`

[next sequence](link)

*anything else*

Rules

You must wait for at least 1 hour before posting an answer, after having posted.
You may not post twice (or more) in a row.
The index number of a sequence is the number after the A part, and with leading zeros removed (e.g. for A000040 the index number is 40)
You can assume that neither the input nor the required output will be outside your languages numerical range, but please don't abuse this by choosing a language that can only use the number 1, for example.
If the length of your submission is greater than 65536 characters long, please provide a link to a way to access the code (pastebin for example).
n will never be larger than 1000, or be out of bounds for the sequence, simply to prevent accuracy discrepancies from stopping a language from competing.
Every 150 (valid) answers, the number of times a language may be used increases. So after 150 solutions have been posted, every language may be used twice (with all previous answers counting towards this). For instance, when 150 answers have been posted, Python 3 may be used twice, but due to the fact that it has already been used once, this means it can only be used once more until 300 answers have been posted.
Please be helpful and post a link to the next sequence to be used. This isn't required, but is a recommendation.
Different versions of languages, e.g. Python 2 and Python 3 are different languages. As a general rule, if the different versions are both available on Try It Online, they are different languages, but keep in mind that this is a general rule and not a rigid answer.
It is not banned, but please try not to copy the code from the OEIS page, and actually try to solve it.
Hardcoding is only allowed if the sequence is finite. Please note that the answer that prompted this (#40) is the exception to the rule. A few answers early in the chain hardcode, but these can be ignored, as there is no good in deleting the chain up to, say, #100.

Answer chain snippet

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body{text-align:left!important;font-family:Roboto,sans-serif}#answer-list,#language-list{padding:10px;/*width:290px*/;float:left;display:flex;flex-wrap:wrap;list-style:none;}table thead{font-weight:700}table td{padding:5px}ul{margin:0px}#board{display:flex;flex-direction:column;}#language-list li{padding:2px 5px;}#langs-tit{margin-bottom:5px}#byte-counts{display:block;margin-left:15px;}#question-author{color:purple;text-shadow: 0 0 15px rgba(128,0,128,0.1);}#label-info{font-weight: normal;font-size: 14px;font-style: italic;color: dimgray;padding-left: 10px;vertical-align: middle; }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script><link rel="stylesheet" type="text/css" href="//cdn.sstatic.net/codegolf/all.css?v=83c949450c8b"><p id="weird-answers"></p><p>Currently waiting on <span id="next"></span></p><span>Search by Byte Count: <input id="search" type="number" min=1 oninput="checkSize(this.value);search(1,this.value)" onclick="document.getElementById('search2').value='';!this.value&&search(0,'')"/> <span id="size-used"></span></span><br><span>Search by Language: <input id="search2" oninput="checkLang(this.value);search(0,this.value)" onclick="document.getElementById('search').value='';!this.value&&search(0,'')"/> <span id="language-used"></span></span><h2>Answer chain <span id="label-info">click a label to sort by column</span></h2><table class="answer-list"><thead><tr><td  onclick="sortby(0)">#</td><td onclick="sortby(1)">Author</td><td onclick="sortby(2)">Language</td><td onclick="sortby(3)">Sequence</td><td onclick="sortby(4)">Size</td></tr></thead><tbody id="answers"></tbody></table><table style="display: none"><tbody id="answer-template"><tr><td>{{PLACE}}</td><td>{{NAME}}</td><td>{{LANGUAGE}}</td><td>{{SEQUENCE}}</td><td>{{SIZE}}</td><td><a href="{{LINK}}">Link</a></td></tr></tbody></table><table style="display: none"><tbody id="language-template"><tr><td>{{LANGUAGE}}</td><td>{{NAME}}</td><td>{{SEQUENCE}}</td><td>{{SIZE}}</td><td><a href="{{LINK}}">Link</a></td></tr></tbody></table>

Comments are not for extended discussion; this conversation has been moved to chat. — Dennis, Commented Oct 31, 2017 at 2:49
Is it OK if a program would need a better floating-point accuracy for the builtin float/double type in order to produce values for larger n? — Maya, Commented Nov 21, 2017 at 15:15
@Giuseppe No, as you're generating the numbers by doing the maths, rather than just placing them into an array/string — caird coinheringaahin g, Commented Dec 15, 2017 at 22:14
@cairdcoinheringaahing In my opinion that's hardcoding the gamma constant. It doesn't work "in theory" for larger numbers. — user202729, Commented Dec 22, 2017 at 12:44

Maya · Accepted Answer · 2017-09-02 17:43:25Z

129. ExtraC, 482 bytes, A000082

import math

int isprime(long n)do
	if(n equals 1) return 0
	if(n equals 2) return 1
	if(not(n modulus 2)) return 0
	long max be sqrt(n)
	for(long i be 2 AND i less max AND increment i)do
		if(not(n modulus (2 times i plus 1))) return 0
	end
	return 1
end

long n be readint plus 1
double r be 1
for(long p be 1 AND p minus 1 less n AND increment p)do
	if(not(n modulus p))do
		if(isprime(p))do
			r be r times (F(1) divide p plus 1)
		end
	end
end
print((int)(r times (n times n)))

One OEIS after another

However!

Input and Output

Formatting

Rules

Answer chain snippet

407 Answers 407

129. ExtraC, 482 bytes, A000082

131. Bean, 116 bytes, A000090

133. CHICKEN Scheme, 643 bytes, A000903

138. Chapel, 689 bytes, A000701

139. Perl 6, 52 bytes, A000689

151. Python 2, 31 bytes, A000125

152. Mathics, 136 bytes, A000031

155. Python 3, 83 bytes, A000262

158. Jelly, 48 bytes, A000376

Explanation

159. Java (OpenJDK 8), 778 bytes, A000048

163. R, 227 bytes, A000448

175. PHP, 122 bytes, A000054

177. Lua, 214 bytes, A000184

179. C (clang), 62 bytes, A000933

180. C (tcc), 63 bytes, A000062

181. C (clang), 276 bytes, A000063

182. Common Lisp, 134 bytes, A000276

183. PostL, 61 bytes, A000134

Walkthrough

186. axo, 177 bytes, A000017

187. SageMath, 321 bytes, A000177

192. Emoji, 326 bytes, A001824

197. D, 347 bytes, A000056

198. Go, 466 bytes, A000347

203. Julia 0.5, 96 bytes, A000050

216. BeanShell, 411 bytes, A000095

230. Cubix, 86 bytes, A001147

285. Squirrel, 151 bytes, A000211

282. Actually, 126 bytes, A011628

Explanation

286. Pyon, 1118 bytes, A000151

288. Swift 4, 186 bytes, A007437

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