! Package inputenc Error: Unicode char Δ (U+394)

A similar problem occurs with


when compiled with pdfLaTeX. It is a (slightly annoying for some) fact that inputenc's UTF-8 config file does not set up all Unicode characters for use with LaTeX. One of the reasons being that fonts in LaTeX contain significantly fewer code points than Unicode and it is not always possible to predict where the glyph resides (if it is present at all).

So as noted in the comments,




would be a solution to your problem. The latter has the advantage that it works in text and math mode, while $\Delta$ will fail if TeX is already in math mode, i.e. with input such as \(ΔR\) (thanks to @egreg for pointing this out in the comments).

The alternative solution of writing

title = {$\Delta\mathrm{R}$ Correction Values for the {Northern} {Indian} {Ocean}},

is what I would prefer here, but I don't know about the conventions in your field, maybe the R should be upright, ... The software you use to export your .bib file should let you use TeX's math mode, so that you won't have to manually edit your .bib file all the time.

Alternatively, you can use of the engines that support Unicode natively, XeLaTeX or LuaLaTeX. With those engines you won't need \DeclareUnicodeCharacter because they can access system fonts directly where LaTeX's character limit does not come into play.

I've been writing in greek using MiKTeX + PdfTeX + TeXstudio in Ubuntu since 2010. For a new laptop I used the same configuration for Windows 10 and the above error occured whenever a greek character was written. After a lot of reading and the problem unsolved, I just tried to change the \usepackage[utf8]{inputenc} with utf8x. And just like that the problem solved. At the same time I use utf in my Ubuntu system and everything is ok!