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A242454

Triangular numbers T such that sum of digits of T is semiprime.

6, 15, 28, 36, 45, 55, 78, 91, 105, 136, 153, 171, 190, 231, 253, 276, 325, 351, 406, 465, 528, 630, 703, 780, 820, 861, 1035, 1081, 1176, 1225, 1275, 1431, 1540, 1596, 1653, 1711, 1770, 2016, 2080, 2211, 2346, 2701, 2775, 2850, 3003, 3160, 3240, 3321, 3403

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OFFSET

1,1

COMMENTS

The n-th triangular number T(n) = n*(n+1)/2.

Triangular numbers with digital sum = p * q, where p and q are primes.

LINKS

K. D. Bajpai, Table of n, a(n) for n = 1..10000

EXAMPLE

a(2) = 15 = 5*(5+1)/2: 1+5 = 6 = 2 * 3 is semiprime.

a(3) = 28 = 7*(7+1)/2: 2+8 = 10 = 2 * 5 is semiprime.

MAPLE

with(numtheory): A242454:= proc()local a, b; a:=x*(x+1)/2; b:=add( i, i = convert((a), base, 10))(a); if bigomega(b)=2 then RETURN (a); fi; end: seq(A242454 (), x=1..100);

MATHEMATICA

Select[Table[n*(n+1)/2, {n, 200}], PrimeOmega[Sum[DigitCount[#][[i]]*i, {i, 1, 9}]] == 2 &]

CROSSREFS

Cf. A000217, A001358, A158195, A118688, A242343.

Sequence in context: A063525 A335267 A349476 * A161777 A117519 A392660

Adjacent sequences: A242451 A242452 A242453 * A242455 A242456 A242457

KEYWORD

nonn,base,less

AUTHOR

K. D. Bajpai, May 15 2014

STATUS

approved

URL: https://oeis.org/A242454

⇱ A242454 - OEIS