# [05AB1E], <s>7</s> 6 bytes **EDIT**: Thanks to @Dennis: Saved a byte because I forgot about the increment operator 8*>t.ï [Try it online!] `n` is triangular if `sqrt(8n + 1)` is an integer How it works 8* # multiply implicit input by 8 > # add one t # sqrt .ï # is integer [05AB1E]: https://github.com/Adriandmen/05AB1E [Try it online!]: https://tio.run/##MzBNTDJM/f/fQsuuRO/w@v//zQA "05AB1E – Try It Online"