%I #14 May 07 2019 17:41:21

%S 1,10,3240,464166,1043290,5740966,335936160,428264011,3529890253,

%T 4015289691,108170400250,821266581691,2216062991691,34830532944360,

%U 2453718234672253,6017966241009003,11215323437683690,214486304402280015,446417240103777645,1792095001020995616

%N Triangular numbers such that moving the last digit to the front produces a square.

%H Giovanni Resta, <a href="/A133198/b133198.txt">Table of n, a(n) for n = 1..100</a>

%e 3240 is a triangular number, 0324 is a square, so 3240 belongs to this sequence.

%p a:=proc(n) local n2, L: n2:=convert((1/2)*n*(n+1),base,10): L:=nops(n2): if type(sqrt((1/20)*n*(n+1)-(1/10)*n2[1]+n2[1]*10^(L-1)),integer)=true then (1/2)*n*(n+1) else end if end proc: seq(a(n),n=1..10^7); # _Emeric Deutsch_, Oct 17 2007

%t Select[Table[n(n + 1)/2, {n, 1000000}], IntegerQ[Sqrt[ FromDigits[ Prepend[Drop[IntegerDigits[ # ], -1], Last[IntegerDigits[ # ]]]]]] &]

%Y Cf. A000217, A000290, A133197.

%K base,nonn

%O 1,2

%A _Tanya Khovanova_, Oct 09 2007

%E More terms from _Emeric Deutsch_, Oct 17 2007

%E Offset corrected and a(15)-a(19) from _Donovan Johnson_, Jan 31 2011

%E a(20) from _Giovanni Resta_, May 11 2016