OFFSET
1,3
LINKS
Michel Marcus, Table of n, a(n) for n = 1..10000
EXAMPLE
153 is a triangular number and the product of digits 15 is also a triangular number.
MAPLE
q:= n-> (l-> issqr(1+8*mul(i, i=l)))(convert(n, base, 10)):
select(q, [seq(i*(i+1)/2, i=0..110)])[]; # Alois P. Heinz, Mar 17 2023
MATHEMATICA
tn=Table[n (n+1)/2, {n, 0, 110}] ; Select[tn, MemberQ[tn, Times@@IntegerDigits[#]]&] (* James C. McMahon, Sep 25 2024 *)
PROG
(Magma) [t: n in [0..110] | IsSquare(8*p+1) where p is &*Intseq(t) where t is (n*(n+1) div 2)]; // Bruno Berselli, Jun 30 2011
(PARI) isok(k) = ispolygonal(k, 3) && ispolygonal(vecprod(digits(k)), 3); \\ Michel Marcus, Mar 17 2023
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Amarnath Murthy, May 02 2001
EXTENSIONS
More terms from Erich Friedman, May 08 2001
Offset 1 from Michel Marcus, Mar 17 2023
STATUS
approved