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A211869 a(n) = Sum_{j=1..n-1} j*(n-j)*b^(j-1) with b = floor(n^2/4)+1. 2
0, 1, 8, 98, 1712, 58985, 2541896, 187337236, 15687030920, 2014736789165, 280434300560320, 55591630021883014, 11642487182670742552, 3294318202343411333713, 969986091740868071844464, 371055858906757952457992360 (list; graph; refs; listen; history; text; internal format)



Equivalently, a(n) is the number having the digits (j*(n-j); j=1..n-1), in base b = floor(n^2/4)+1.

From R. J. Cano, Mar 03 2018: (Start)

If a(n) were converted to the base 1+floor(n^2/4)=A033638(n) then a palindrome would be obtained. Such palindrome is related to A215940(n!);

a(7)=2541896 and A033638(7)=13, giving the palindrome "6ACCA6". Such palindrome cannot be converted directly to decimal, but it might be defined instead from these digits the polynomial f(t)= 6*t^5 +10*t^4 +12*t^3 +12*t^2+10*t^1+6*t^0, then evaluating for t=10, we get f(10)=713306=A215940(7!). 713306 clearly looks distinct than "6ACCA6". f(11) and f(12) respectively are 1130256 with "7021A6", and 1722942 with "6B10A6". Now evaluating f(14) we get 3646530 and if converted to base 14 it yields "6ACCA6". The same happens with f(15) converted to base 15, f(16) converted to Hexadecimal, and also in general for f(y) converted to base y, if it were provided that y>=13.

Here A033638(n) gives the lower bound for the infinite set of bases where this behavior can be observed. For simplicity it is chosen the base A033638(n) when defining this sequence, although what we actually want is to keep the pattern generated by the products j*(n-j). (End)

This sequence together with A033638 and A215940 demonstrates the connection among permutation sets and palindromes obtained by symmetric products. - Alexander R. Povolotsky, Feb 08 2013


R. J. Cano, Table of n, a(n) for n = 1..57

R. J. Cano, Additional information on this sequence.

R. J. Cano, A211869: Gauss taught me that.


a(n) = Sum_{j=1..n-1} j*(n-j)*A033638(n)^(n-1-j).


For n=5, the four products are 1*4 = 4, 2*3 = 6, 3*2 = 6, 4*1 = 4, giving the base-7 concatenation 4664. In base 10, this is a(5) = 1712.

For a(6) we have that 1+floor(6^2/4) = 10 so there is no need of converting the concatenation to decimal. By definition the products are j*(n-j) for j in 1..5: 1*(6-1) = 5 = 5*(6-5), 2*(6-2) = 8 = 4*(6-2), 3*(6-3) = 9 so the result is a(6)=58985.


(PARI) a(n, base=1+n^2\4)=sum(j=1, n-1, j*(n-j)*base^(n-1-j));


Cf. A033638, A215940, A083449, A019566, A002113.

Sequence in context: A199029 A228329 A228794 * A159232 A297958 A298774

Adjacent sequences:  A211866 A211867 A211868 * A211870 A211871 A211872




R. J. Cano, Feb 02 2013



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Last modified February 28 14:16 EST 2021. Contains 341707 sequences. (Running on oeis4.)