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A113940
Triangular numbers that are also brilliant (A078972).
3
6, 10, 15, 21, 253, 703, 1081, 1711, 1891, 2701, 3403, 25651, 34453, 38503, 49141, 60031, 64261, 73153, 79003, 88831, 104653, 108811, 114481, 126253, 146611, 158203, 171991, 188191, 218791, 226801, 258121, 269011, 286903, 351541, 371953, 385003, 392941
OFFSET
1,1
COMMENTS
Smallest value where each factor has n digits for n = 1, 2, 3, 4, 5, are: 6 = 2 * 3; 253 = 11 * 23; 25651 = 113 * 227; 2035153 = 1009 * 2017; 202457503 = 10061 * 20123. [From Jonathan Vos Post, Apr 04 2009]
LINKS
FORMULA
A000217 INTERSECTION A078972. Subset of A068443. [From Jonathan Vos Post, Apr 04 2009]
EXAMPLE
253 = T(22) and 253 = 11*23 is brilliant.
MATHEMATICA
brilQ[n_]:=Module[{fin=FactorInteger[n]}, Total[Transpose[fin][[2]]]==2&& Length[Union[IntegerLength[Transpose[fin][[1]]]]]==1]
Intersection[Accumulate[Range[850]], Select[Range[362000], brilQ]] (* Harvey P. Dale, Feb 06 2011 *)
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Giovanni Resta, Jan 31 2006
STATUS
approved