# [seqfan] A Remarkable Eigenfunction - A179497

Paul D Hanna pauldhanna at juno.com
Sun Aug 1 15:58:48 CEST 2010

```SeqFans,

Consider the eigenfunction described by:

E.g.f. satisfies: A(A(x))^2 = A(x)^2 * A'(x)

[http://www2.research.att.com/~njas/sequences/A179497].

Explicitly, the series begins:

A(x) = x + 2*x^2/2! + 18*x^3/3! + 312*x^4/4! + 8240*x^5/5! + 297000*x^6/6! + 13705776*x^7/7! + 776778688*x^8/8! + 52511234688*x^9/9! + 4143702216960*x^10/10! +..

Let A_n(x) denote the n-th iteration of A(x) where A_{n+1}(x) = A_n(A(x)) with A_0(x)=x;

then the pattern continues:

[A_3(x)]^2 = A(x)^2 * A_2'(x),

[A_4(x)]^2 = A(x)^2 * A_3'(x),

[A_5(x)]^2 = A(x)^2 * A_4'(x), ...

so that the iterations of A(x) satisfy:

(*) [A_{n+1}(x)]^2 = A(x)^2 * A_n'(x)   for all n.

I find formula (*) to be remarkable, even though the proof for n>0
is an easy exercise in induction and the chain rule.

Surely A(x) must have some other nice properties.

What else can be said about A(x)?

Paul

```

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