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
MATHEMATICA
Select[Table[n*(n+1)/2, {n, 200}], PrimeOmega[Sum[DigitCount[#][[i]]*i, {i, 1, 9}]] == 2 &]
CROSSREFS
KEYWORD
nonn,base,less
AUTHOR
K. D. Bajpai, May 15 2014
STATUS
approved