A portrait of Fanny Mendelssohn dated 1847. (Library of Congress, Music Division)

“Easter Sonata” — a complex four-movement piano composition from 19th century Germany — could only have been written by Felix Mendelssohn.

Or so thought many of the archivists, scholars and musicians who encountered it. The sonata was “masculine,” “violent” and “ambitious,” all the hallmarks of the celebrated Romantic era composer.

Written in 1829, the manuscript of “Easter Sonata” was considered “lost” for more than 140 years, until the original turned up in a French book shop bearing the signature “F Mendelssohn.” The collector who bought it concluded the “F” stood for Felix.

It took yet another four decades and a lot of clever musicological sleuthing, but in 2010 a Duke University graduate student revealed what some had suspected all along: “Easter Sonata” was not written by Felix Mendelssohn, but by his sister, Fanny Mendelssohn, herself a musical prodigy.

On Wednesday, in honor of International Women’s Day, “Easter Sonata” was performed under Fanny Mendelssohn’s name for the first time in a public concert hall, bringing Fanny and her widely recognized masterpiece out of her brother’s shadow after 188 years.

Pianist Sofya Gulyak performed the roughly 20-minute composition at the Royal College of Music in London. Among those in the audience was Fanny’s great-great-great granddaughter, Sheila Hayman, a filmmaker and novelist who discussed the story behind “Easter Sonata” with the BBC and wrote about it for the Guardian.

“She was an amazing woman,” Hayman said of Fanny Mendelssohn, “who persevered despite complete discouragement.”

Fanny was born to a wealthy German family in 1805, three years before Felix. Both children displayed musical talents early on. By age 14, Hayman wrote in the Guardian, Fanny could play all of Bach’s 48 preludes and fugues by heart. But her parents didn’t allow her to pursue music as a career, believing it would be indecent for a young woman.

Her father’s response was, in effect, “Very nice dear, but don’t forget you’re a girl, so you can forget about taking this stuff up publicly,” Hayman wrote.

Felix’s musical ambitions, on the other hand, were encouraged. In 1829, he set off on a tour throughout Europe to play and study composers, while Fanny stayed home.

In a diary entry written the day Felix left on his musical journey, Fanny wrote, “I played my Easter Sonata.” Written at 22 years old, the piece was among about 500 she composed before she died in 1847 of a stroke at 41 years old. Felix died less than six months later, with some experts saying his death was caused by sadness over losing his sister. The two were said to have been inseparable throughout their lives.

The manuscript of “Easter Sonata” remained hidden until a record collector and producer named Henri-Jacques Coudert reportedly found it in a Paris bookshop in 1970. Convinced it was a newly unearthed masterpiece by Felix, Coudert brought the manuscript to French pianist Eric Heidsieck, who made the first known recorded version of “Easter Sonata” in 1973.

It’s not clear when people started to wonder if Fanny was the true composer. Her diary entry suggested so, but for many years it was impossible to verify.

A 1992 biography of Fanny says the sonata was “automatically” ascribed to her brother, even though it was not recorded in any of Felix’s catalogues. The author of the biography, Françoise Tillard, said the manuscript’s owner at the time would “not allow anyone to consult the original.”

“Even if one could recognize the hand of Felix or Fanny, there would be nothing to prove that one of them had not recopied the other’s work,” Tillard wrote, adding, somewhat prophetically, “the future will provide an answer to the questions raised by this piece.”

That moment came in 2010, when Angela Mace Christian, then a Duke University graduate student, tracked the manuscript down in a private archive in Paris. Christian told Smithsonian Magazine she had followed a “documentary trail” of letters and diary entries.

“I was able to see that it was in [Fanny’s] handwriting,” Christian said. She added that the manuscript contained page numbers that were missing from a different manuscript known to have been composed by Fanny.

Those two clues were the “major factors pointing to the identification” that the sonata was hers, Christian said.

Thanks to Christian’s work, “Easter Sonata” is now broadly accepted as Fanny’s. But some skeptics remain, including Coudert. “It can’t be by Fanny,” he reportedly told Christian.

During the performance Wednesday at the Royal College of Music, pianist Sofya Gulyak played a rousing rendition of the composition, at long last under Fanny’s name. In remarks beforehand she praised Fanny’s “distinctive” voice, saying she had drawn from contemporaries such as Beethoven while achieving a style totally her own.

Gulyak said the composition’s “chorale” — the hymn-like final movement — served as a kind of coda to the story behind “Easter Sonata.”

It was “like an ascension,” Gulyak said, “like an enlightenment in the end,” which “says a lot about her spirit.”

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