A079654 Triangular numbers using only the straight digits 1, 4 and 7.

1, 171, 741, 1711, 117144471, 417417171, 7417744447141, 177141144447177414711441, 1417711714147441711474771771, 144114474147744777714441111111771711, 111411774474177744717747747477177774111

Offset: 1

Author

Shyam Sunder Gupta, Jan 23 2003

Keywords

• base
• nonn

Comments

Probably finite.

a(12) > 10^40 if it exists. - Tyler Busby, Mar 22 2023

Links

• Giovanni Resta, Tridigital Triangular Numbers.

Crossrefs

Cf. A000217, A028373, A053899, A119127. See A119033 for a table of cross-references.

Programs

Mathematica

Do[ If[ Union[ Join[ IntegerDigits[n(n + 1)/2], {1, 4, 7}]] == {1, 4, 7}, Print[n(n + 1)/2]], {n, 0, 3*10^7}]

Formula

a(n) = A000217(A119127(n)). - Tyler Busby, Mar 31 2023

Extensions

Edited and extended by Robert G. Wilson v, Jan 24 2003

More terms from Giovanni Resta, May 10 2006

a(10)-a(11) from Tyler Busby, Mar 22 2023