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%I #8 Mar 30 2012 18:36:00
%S 0,1,4,21,10,69905,2451,1432028818087,18166035,72719023,
%T 316138782904777094553940,14747027341648947707567131341794256650861,
%U 1801710500115237891740015,37089922913701070144219234699757985931372002942260401897575393
%N Smallest k such that k*n!! +1 is an n-th power.
%e a(3) = 21 because 21*3!! + 1 = 21*3+1 = 64 = 4^3.
%t a={}; Do[k = 2; While[ !IntegerQ[(k^n - 1)/n!!], k++ ]; AppendTo[a, (k^n-1)/n!!], {n, 0, 14}]; a
%Y Cf. A006882.
%K nonn
%O 0,3
%A _Michel Lagneau_, Dec 19 2011
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