This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
a(1)=1 => a(2)=3*1+1=4 because a(1) is odd => a(3)=4/2=2 because a(2) is even => a(4)=3*2+0=6 because a(3) is even but a(3)/2 is already in the sequence.
a[0] = 0; a[1] = 1; a[n_] := a[n] = With[{b = a[n-1]}, If[EvenQ[b] && FreeQ[Array[a, n, 0], b/2], b/2, 3 b + Mod[b, 2]]];
Table[a[n], {n, 0, 100}] (* Jean-François Alcover, Jun 20 2019 *)
a[1] = 1; a[n_] := a[n] = If[Divisible[a[n-1],3], a[n-1]/3, 4*a[n-1]+1]; Array[a, 50] (* Amiram Eldar, Jul 12 2021 *)
a(n) = if (n==1, 1, my(x=a(n-1)); if (x % 3, 4*x+1, x/3)); \\ Michel Marcus, Aug 12 2021
// generates all the numbers in the sequence before it first surpasses 1 billion
int n;
void setup() {
n = 1;
noLoop();
}
void draw() {
print(n + ",");
while (true) {
if (n % 3 == 0) {
n /= 3;
} else {
n *= 4; n++;
}
print(n);
if (n == 1 || n >= 600000000) {
break;
} else {
print(", ");
}
}
}
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