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

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

Offset

1,2

Comments

Probably finite.

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

Links

Table of n, a(n) for n=1..11.

Giovanni Resta, Tridigital Triangular Numbers.

Formula

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

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}]

Crossrefs

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

Sequence in context: A043407 A031691 A158220 * A220869 A251329 A221754

Adjacent sequences: A079651 A079652 A079653 * A079655 A079656 A079657

Keyword

base,nonn

Author

Shyam Sunder Gupta, Jan 23 2003

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

Status

approved