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EXAMPLE
| a(2) = [x^2] (1 + 2x)*(1 + 2x + 3x^2) = [x^2] (1 + 4x + 7x^2 + 6x^3) = 7.
a(3) = [x^3] (1 + 2x)*(1 + 2x + 3x^2)*(1 + 2x + 3x^2 + 4x^3)
= [x^3] (1 + 6x + 18x^2 + 36x^3 + 49x^4 + 46x^5 + 24x^6) = 36.
This sequence is a diagonal in the triangle of successive products:
(1);
1,(2);
1,4,(7),6;
1,6,18,(36),49,46,24;
1,8,33,94,(204),354,497,562,501,326,120;
1,10,52,188,528,(1222),2406,4102,6116,7996,9132,9014,7541,5116,2556,720; ...
Lower diagonals are convolutions with this sequence and A006013:
[1,4,18,94,528,3106,18798,115964, ...] = A006013 * A129261;
[1,6,33,188,1105,6660,40888,254510,...]= A006013^2 * A129261.
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