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A219253
Numbers k such that k = n-th triangular number and last digits of k match n.
0
1, 15, 325, 195625, 43959376, 4106490625, 396606890625, 25271617109376, 83084112890625, 22661209212890625, 1596879961787109376, 3344565630081787109376, 1795096118003159918212890625
OFFSET
1,2
COMMENTS
Called trimorphic numbers.
Corresponding values of n: 1, 5, 25, 625, 9376, 90625, 890625, 7109376, 12890625, 212890625, 1787109376, 81787109376, 59918212890625. - Zak Seidov, Apr 12 2013
REFERENCES
James J. Tattersall, Elementary Number Theory in Nine Chapters, Second Edition, Cambridge University Press, p. 44, problem no. 9.
EXAMPLE
15 is in the list since 15 = 5th triangular number and last digit of 15 is 5. Similarly 325 = 25th triangular number and last two digits of 325 gives 25.
MATHEMATICA
t = {}; Do[k = n*(n + 1)/2; p = Length[IntegerDigits[n]]; If[FromDigits[Take[IntegerDigits[k], -p]] == n, AppendTo[t, k]], {n, 240000000}]; t
CROSSREFS
Sequence in context: A077738 A068124 A094399 * A119296 A180779 A196666
KEYWORD
nonn,base
AUTHOR
Jayanta Basu, Apr 11 2013
EXTENSIONS
a(11)-a(13) from Zak Seidov, Apr 12 2013
STATUS
approved