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A055235 Sums of two powers of 3. 7
2, 4, 6, 10, 12, 18, 28, 30, 36, 54, 82, 84, 90, 108, 162, 244, 246, 252, 270, 324, 486, 730, 732, 738, 756, 810, 972, 1458, 2188, 2190, 2196, 2214, 2268, 2430, 2916, 4374, 6562, 6564, 6570, 6588, 6642, 6804, 7290, 8748, 13122, 19684, 19686, 19692, 19710 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

T. D. Noe, Rows n = 0..100 of triangle, flattened

FORMULA

a(n+1) = 3^(n-trinv(n)*(trinv(n)+1)/2)+3^trinv(n), where trinv(n) = floor((sqrt(1+8*n)-1)/2) = A003056(n) and n-trinv(n)*(trinv(n)+1)/2 = A002262(n).

Regarded as a triangle, T(n, k) = 3^n + 3^k, because 3^n + 3^n < 3^(n+1) + 3^0 for all n > 0.

MATHEMATICA

mx = 10; Sort[Flatten[Table[3^x + 3^y, {y, 0, mx}, {x, 0, y}]]] (* Vladimir Joseph Stephan Orlovsky, Apr 20 2011 *)

f[n_] := Block[{t = Floor[(Sqrt[1 + 8 (n - 1)] - 1)/2]}, 3^(n - 1 - t*(t + 1)/2) + 3^t]; Array[f, 49] (* Robert G. Wilson v, Oct 08 2011 *)

PROG

(PARI) for( n=0, 5, for(k=0, n, print1(3^n+3^k", ")))

A055235(n)={ my( t=(sqrtint(8*n-7)-1)\2); 3^t+3^(n-1-t*(t+1)/2) }  \\ M. F. Hasler, Oct 08 2011

CROSSREFS

Cf. A052216.

Sequence in context: A316460 A065385 A244052 * A083887 A064374 A000885

Adjacent sequences:  A055232 A055233 A055234 * A055236 A055237 A055238

KEYWORD

easy,nonn,tabl

AUTHOR

Henry Bottomley, Jun 22 2000

EXTENSIONS

Formula corrected by M. F. Hasler, Oct 08 2011

STATUS

approved

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Last modified September 25 11:05 EDT 2018. Contains 315389 sequences. (Running on oeis4.)